Earth life 'may have come from Mars'

Earth life 'may have come from Mars'

New research, presented at a major scientific conference, supports the view that the conditions on Mars were more favourable for kick-starting life’s building blocks than Earth, suggesting that life started on Mars and came to Earth on a meteorite.

Details of the theory were outlined by Professor Steven Benner at the Goldschmidt Meeting in Florence, Italy. The evidence is based on how the first molecules necessary for life were assembled. Scientists have long wondered how atoms first came together to make up the three crucial molecular components of living organisms: RNA, DNA and proteins.

The molecules that combined to form genetic material are far more complex than the primordial soup of carbon-based chemicals thought to have existed on the Earth more than three billion years ago, and RNA is thought to have been the first of them to appear.

The minerals most effective at forming RNA, boron and molybdenum, would not have existed in a sufficient form or quantity in the early Earth, but would have been more abundant on Mars, according to Professor Benner. This could suggest that life started on the Red Planet before being transported to Earth on meteorites.

"This form of molybdenum couldn’t have been available on Earth at the time life first began, because three billion years ago, the surface of the Earth had very little oxygen, but Mars did,” said Professor Benner. "It’s yet another piece of evidence which makes it more likely life came to Earth on a Martian meteorite, rather than starting on this planet."

Meteorites from Mars have been arriving on Earth throughout our planet's history so Benner’s theory is certainly not implausible.

"The evidence seems to be building that we are actually all Martians; that life started on Mars and came to Earth on a rock," he commented.


    The Search For Extraterrestrial Life: A Brief History

    If (or, as some would say,_ when_) humans make contact with alien intelligence, the scientists who devote their careers to the search will be our first point of contact. Here, we look at the history of one of humankind's most persistent fascinations

    For as long as humans have looked to the night sky to divine meaning and a place in the universe, we have let our minds wander to thoughts of distant worlds populated by beings unlike ourselves. The ancient Greeks were the first Western thinkers to consider formally the possibility of an infinite universe housing an infinite number of civilizations. Much later, in the 16th century, the Copernican model of a heliocentric solar system opened the door to all sorts of extraterrestrial musings (once the Earth was no longer at the center of creation and was merely one body in a vast cloud of celestial objects, who was to say God hadn’t set other life-sustaining worlds into motion?) While that line of thinking never sat well with the church, speculation about alien life kept pace with scientific inquiry up through the Enlightenment and on into the twentieth century.

    But it wasn’t until the close of the 1950s that anyone proposed a credible way to look for these distant, hypothetical neighbors. The space age had dawned, and science was anxious to know what lay in wait beyond the confines of our thin, insulating atmosphere. The Russians had, in 1957 and 1958, launched the first three Sputnik satellites into Earth orbit the United States was poised to launch in 1960 the successful Pioneer 5 interplanetary probe out toward Venus. We were readying machines to travel farther than most of us could imagine, but in the context of the vast reaches of outer space, we would come no closer to unknown planetary systems than if we’d never left Earth at all.

    Our only strategy was to hope intelligent life had taken root elsewhere and evolved well beyond our technological capabilities—to the point at which they could call us across the empty plains of space. Our challenge was to figure out which phone might be ringing and how exactly to pick it up. And so it was in mid-September of 1959 that two young physicists at Cornell University authored a two-page article in Nature magazine entitled “Searching for Interstellar Communications.” With that, the modern search for extraterrestrial life was born, and life on Earth would never again be the same.

    _Launch the gallery to see how the search began and where it will take us next._

    The Birth of SETI

    Giuseppe Cocconi and Philip Morrison—two physicists at Cornell—began their 1959 article in Nature magazine quite frankly: we can’t reliably estimate the probability of intelligent life out in the universe, but we can’t dismiss the possibility of it either. We evolved and we’re intelligent, so wouldn’t it stand to reason that alien civilizations could arise on planets around other sun-like stars? In all likelihood, some of those civilizations would be older and more advanced than ours and would recognize our Sun as a star which could be host to life, with whom they would want to make contact. The central question of the paper was then: how would the beings send out their message? Electromagnetic waves were the most logical choice. They travel at the speed of light and would not disperse over the tremendous distances between stars. But at which frequency? The electromagnetic spectrum is far too wide to scan in its entirety, so they made an assumption that has remained central to SETI research ever since. They would listen in at 1420 MHz, which is the emission frequency of hydrogen, the most abundant element in the universe. They reasoned it was the one obvious astronomical commonality we would share with an unknown civilization and that they would recognize it too.

    The Drake Equation

    Only a few years later, in 1961, the nebulous assumptions Cocconi and Morrison parlayed in their article got a bonafide mathematical equation. Frank Drake [with equation, at left], along with a handful of other astronomers and scientists (including Carl Sagan) met in Green Bank, West Virginia to hash out the formula and variables necessary to make an educated guess at just how many intelligent civilizations might be living in our galaxy. As it turns out, assigning numbers to nebulous assumptions nets you an answer with enough variance to make you wonder if you were really clarifying those assumptions in the first place. The group came up with a range from less than a thousand to nearly a billion. You might think the formula would have been refined over the years, but that is not the case. It has held up surprisingly well (though, for such a nebulous equation “held up” is a relative phrase). Data collected since the 1960s, which can be used to support the original estimates of measurable quantities like how often sun-like stars form and how many of those stars have planets, has proven those estimates to have been relatively accurate. The rest of the variables will never be quantified, such as what fraction of life evolves to become intelligent and what the average lifetime of an intelligent civilization is. Still, the equation has served as a focal point for SETI investigations over the years and continues to be valuable framework, however controversial.

    Astrobiology

    When we aren’t looking for beacons from intelligent life forms in deep space, our studies in the realm of extraterrestrial life turn inward. How did life on Earth originate? How did intelligent life on Earth originate? These are two of the key questions at the heart of the interdisciplinary field known as astrobiology. While much of the work of astrobiologists can be speculative—extrapolating what may be elsewhere from what we know to be on Earth—that speculation must first come from solid research on what we see around us. From what we know of life, it’s generally assumed that extraterrestrials will be carbon-based, will need the presence of liquid water, and will exist on a planet around a sun-like star. Astrobiologists use those guidelines as the starting point for looking outward. Of course, the discipline includes traditional astronomy and geology as well. These are necessary fields for understanding where we should be looking for life outside of Earth and which properties we should seek when studying stars and their planets. While astrobiologists are looking deep into space for evidence of all these things, the largest single object of study is currently right in our literal backyard: Mars.

    Life on Mars

    We can safely assume we won’t find any little green men on Mars. Likely, too, that we won’t come upon any grey humanoid beings with almond-shaped, black onyx eyes and elongated skulls. But the chances are good that we could find alien life in the form of bacteria or extremophiles, which are bacteria-like organisms that can live in seemingly inhospitable environments. We have sent a variety of probes, landers, and orbiters to Mars, from the Mariner 4 in 1965 to the Phoenix mission, which landed in the planet’s polar region this past May and continues to send back a tremendous amount of data. What we’re looking for first and foremost is water, whether liquid or ice, one of the three keys to extraterrestrial life. “I think it’s probably the best bet for life nearby,” says Dr. Seth Shostak, Senior Astronomer at the SETI Institute. “You could argue that some of the Jovian moons—Europa, Ganymede, Callisto—or Titan and Enceladus, these moons of Saturn, might have life. Even Venus might have life in the upper atmosphere. All those are possible because all those are worlds that might have liquid water. Mars you can see things on the ground, you can go dig around in the dirt, so we have a lot of people who worry about Mars. They’re looking for life and we hope it’s one of the right places.” Even without visiting the red planet, scientists have been poring over meteorites from Mars, tracing fine lines in the rocks which they have theorized were left by bacteria. The trails contain no DNA, however, so the theory remains unproven.

    Project Cyclops

    Cocconi and Morrison’s 1959 article about a systematic search for intelligent life took over a decade to filter through the various arteries of the burgeoning exploratory programs at NASA before it took the shape of a formalized research team. Known as Project Cyclops, the team and its resulting report document were the first large-scale investigation into practical SETI. It outlined many of the same conclusions Cocconi and Morrison reached: that SETI was a legitimate scientific undertaking and that it should be done in the low frequency end of the microwave spectrum. What was not advantageous to the endeavor was the report’s scope of cost, scale, and timeline. It called for a budget of 6 to 10 billion dollars to build and maintain a large radio telescope array over 10 to 15 years. It also made note of the fact that the search would likely take decades to be successful, requiring “a long term funding commitment.” Certainly that was the project’s death knell, and indeed, funding for Project Cyclops was terminated shortly after the report was issued. It would be 21 years before NASA finally implemented a working SETI program, called the High Resolution Microwave Survey Targeted Search (HRMS). But, like its predecessor, it would be exceptionally short-lived, losing operational funding nearly a year to the day later in October of 1993.

    Pioneer Plaques (Pioneers 10 and 11)

    As the search for signals from intelligent life was gaining credibility in the late 60s and early 70s, plans were at the same time underway to send out messages of our own. The mission of the Pioneer 10 and 11 spacecrafts in 1973 was to explore the Asteroid Belt, Jupiter, and Saturn after that point, they would continue their trajectories past Pluto and on into the interstellar medium. With that distant course in mind, Carl Sagan was approached to design a message that an alien race might decipher should either craft be one day intercepted. Together with Frank Drake, Sagan designed a plaque [left] which shows the figures of a man and woman to scale with an image of the spacecraft, a diagram of the wavelength and frequency of hydrogen, and a series of maps detailing the location of our Sun, solar system, and the path the Pioneer took on its way out. It was a pictogram designed to cram the most information possible into the smallest space while still being readable, but was criticized for being too difficult to decode. While the Pioneer 10 became the first man-made object to leave the solar system in 1983, it will be at least two million years before either reaches another star.

    Arecibo Message

    Since the advent of powerful radio and television broadcasting antennas, the Earth has been a relatively noisy place. News and entertainment signals have for decades been bounced off the upper reaches of our atmosphere, with plenty leaking out every which way into space. Those not pulled in by our TVs could one day reach distant stars, in a kind of scatter-shot bulletin announcing our presence through I Love Lucy and Seinfeld. (An unintended consequence of satellite and cable transmissions is the gradual end of high-powered radio signals, making the Earth a much more difficult place to “hear” for anyone listening in.) In 1974, however, a formalized message was beamed out from the newly renovated Arecibo telescope in Puerto Rico. Again designed by Drake and Sagan, the binary radio signal [left] held within it information about the makeup of our DNA and pictographs of a man, the solar system, and the Arecibo telescope. The broadcast was ultimately more a symbolic demonstration of the power of the new Arecibo equipment than a systematic attempt at making contact with ET. The star cluster to which the signal was sent was chosen largely because it would be in the sky during the remodeling ceremony at which the broadcast was to take place. What’s more, the cluster will have moved out of range of the beam during the 25,000 years it will take the message to get there. It was an indication that we would likely not be in the business of sending messages, as it was much cheaper and easier to use radio telescopes to listen, rather than talk. But Sagan and Drake would have one more shot at deep space communications in 1977 with the launch of the Voyager probes.

    Voyager Golden Records (Voyagers 1 and 2)

    While the Pioneer Plaques were devised during a compressed timeline of three weeks and the Arecibo Message was sent according to the timetable of a cocktail party, the Voyager Golden Records were meant to be a brief compendium of the human experience on Earth and so were given the time and NASA committee resources to make them exceptional. The golden records contain 115 video images, greetings spoken in 55 languages, 90 minutes of music from around the world, as well as a selection of natural sounds like birdsongs, surf, and thunder. Again, hydrogen is the key to unlocking the messages the same lowest states diagram which appeared on the Pioneer Plaques is here describing the map locating the sun in the Milky Way. It informs the discoverer how to play the record, at what speed, and what to expect when looking for the video images. It’s even electroplated with a sample of Uranium so that it might be half-life dated far in the future. Since the Voyager probes are moving much more slowly than radio waves, it will take them nearly twice as long as the Arecibo Message to reach their target stars. Even then, after 40,000 years, they’ll only come to within a light-year and a half away. That’s equivalent to about 130 times the distance Pluto is from our sun. It’s an understatement to say that any of these beacons we’ve sent have a very long shot of reaching an intelligent civilization, if one exists and happens to exist in the general direction in which they’re traveling. It’s a reminder of just how inhuman the scales become when we measure the distances in outer space and try to find ways to best them in our search for others like us.

    Meteorites

    As astrobiologists contemplate the origin of life on our planet, they often look to external sources for the ingredients. Asteroids, comets, and meteorites are the ancient relics of the birth of our solar system. They’re the icy and rocky bits zipping around, crashing into each other and into moons and planets, delivering minerals, water, and, as it turns out, amino acids. It’s amino acids—twenty in particular—that are the basis for protein formation, which in turn are the basis for life. So far, we have only discovered eight of those twenty in meteorites. Where the others formed may be one of the secrets to life on Earth and possibly life on other planets. In the historic 1953 Miller-Urey experiment, a concoction of water and the elements of a primordial atmosphere were mixed and electrified to simulate the soup of early Earth. At the end of a week, amino acids had been formed. Of course, there are myriad other unknown processes which need to occur to take us from amino acids to life. As Dr. Seth Shostak of the SETI Institute put it, “just because you have a brickyard in your backyard doesn’t mean you’re going to see a skyscraper appear one day.”

    Extremophiles

    Studying extremophiles may be as close as we get to studying aliens before we actually find extraterrestrial life. Extremophiles are organisms which live in environments inhospitable to all other life as we know it. Some may even physically require these extremes of temperature, pressure, and acidity to survive. They have been found miles under the ocean’s surface and at the tops of the Himalayas, from the poles to the equator, in temperatures ranging from nearly absolute zero to over 300 degrees Fahrenheit. Most extremophiles are single-celled microorganisms, like the domain Archea, whose members may account for 20 percent of the Earth’s biomass. These are the kind of creatures we would expect to find on Mars. But maybe the most alien-like of all extremophiles known to man are the millimeter-long tardigrades, or water bears [left], so called because they have the ability to undergo cryptobiosis. It’s an extreme form of hibernation during which all metabolic activity comes to a near complete standstill and allows the animals to survive everything from massively fatal doses of radiation (to humans) to the vacuum of space. Some argue this suspended state doesn’t technically qualify tardigrades as extremophiles because they aren’t thriving in these environments, they are merely protecting themselves from death. Nevertheless, the more we understand about these organisms’ ability to withstand environments thought to be inhospitable to life, the closer we may come to discovering them outside our planet.

    The Wow! Signal

    Though NASA killed Project Cyclops before it begin, that didn’t mean no one was listening in on the cosmos during the 1970s. Several small-scale SETI projects existed around the country and around the world, many of them operating on university equipment. One of the most prominent—and longest running on SETI work—was the Big Ear radio telescope operated by Ohio State University. The Big Ear was the size of three football fields and looked like a giant silver parking lot with scaffolding for enormous drive-in movie screens at either end. On August 15, 1977, the Big Ear received a signal for 72 seconds which went so far off the charts that the astronomer monitoring the signal print-outs circled the alphanumeric sequence and wrote “Wow!” in the margin. The pattern of signal rose and fell perfectly in sync with the way the telescope was moving through its beam of focus. As it came into view, it became progressively stronger. If the signal had been terrestrial, it would have come in at full strength. It was the best anyone had yet seen. Unfortunately, two other attributes of the Wow! signal worked against it being a legitimate ET beacon. The first had to do with how the Big Ear collected radio waves. It used two collectors, spaced three minutes apart, side-by-side. Any signal caught by the first would have to be caught by the second three minutes later, but that wasn’t the case with the Wow! signal. Only the first horn caught it. Even more discouraging, it hasn’t been seen since. Many operations have tried, using more sensitive equipment and focusing for much longer on the alleged source to no avail.

    Project Phoenix and the SETI Institute

    NASA’s High Resolution Microwave Survey Targeted Search really never stood a chance. Just as soon as it got underway in 1992, members of Congress began to hold it up as a waste of taxpayer money and deride it as frivolous (even though it accounted for less than 0.1 percent of NASA’s annual operating budget). When it was cancelled in the fall of 1993, the SETI Institute moved in to save the core science and engineering team and continue the work under its auspices. It was renamed Phoenix Project and ran for a decade from 1994 to 2004 entirely on funding from private donations. The project used a variety of large telescopes from around the world to conduct its research, observing nearly 800 stars in the neighborhood of up to 240 light years away. After sweeping through a billion frequency channels for each of the 800 stars over the course of 11,000 observation hours, the program ended without having detected a viable ET signal.

    [email protected] at UC Berkeley

    If you know anything about SETI and are of a certain age, chances are you know about it because of the [email protected] project at the University of California, Berkeley. [email protected] was one of the earliest successful distributed computing projects. The concept behind these projects works like this: researchers who have tremendous amounts of raw data and no possible way to process it all themselves split it into tiny chunks and subcontract it out. When you sign up for a distributed project, your computer gets one of these chunks and works on it when it’s not busy, say when you leave your desk to get a coffee or take lunch. When your computer finishes, it sends that chunk back and asks for another. Taken as a whole, distributed computing projects are able to harness an otherwise impossible amount of processing power. The [email protected] project currently gets all its data from the Arecibo radio telescope. It piggybacks on other astronomical research by collecting signals from wherever the telescope happens to be pointed during the brief moments when it is not being used. While the project has not yet detected an ET signal, it has been tremendously beneficial in proving that distributed computing solutions do work and work well, having logged over two million years of aggregate computing time.

    Vatican Observatory

    Galileo wasn’t the only astronomer to have been accused by the Catholic Church of heresy for his beliefs in a heliocentric universe. Giordano Bruno was burned at the stake in the 16th century for arguing that every star had its own planetary system. How far the Church has come, then, with the announcement earlier this year from the Vatican Observatory that you can believe in God and in aliens and it isn’t a contradiction in faith. The Reverend Joes Gabriel Funes, director of the Observatory, says the sheer size of the universe points to the possibility of extraterrestrial life. Because an ET would be part of creation, they would be considered God’s creatures.

    Extrasolar Planets

    If it could be said a single discovery kick-started the search for extrasolar planets, it would be that of 51 Pegasi b [left], in 1995. It was the first extrasolar planet to be found orbiting a normal star and was discovered using the same Doppler effect we experience every day when a siren passes by us at high speed. It was a popular news story at the time—finally we had confirmation that just maybe our solar system was not unique. Since that day, we’ve learned how common, in fact, our system may be. As of early June 2008, the number of confirmed extrasolar planets is nearly 300 it climbs exponentially every year as our technologies for detection grow more sophisticated. To be sure, the vast majority of these planets are gas giants in close, short orbits around their stars—not the kind of celestial bodies on which we expect to find life. That’s not to say that Earth-like, terrestrial planets aren’t out there as well. It’s just that the gas giants are much easier to “see” when we go looking because they tend to zip around their parent stars in a matter of days. We watch those stars for variations in the way they give off light, but don’t actually spot the planets themselves because they are so many magnitudes dimmer than their parent stars. Gas giants are large enough and move quickly enough to produce a noticeable effect on their stars from here on Earth, but for a planet similar to Earth’s size, that’s not the case. In order to find an Earth-sized planet, we would need to watch a star nonstop for years on end and be able to detect the slightest change in brightness as the planet passed in front of it (known as a transit). Fortunately for SETI enthusiasts, NASA has just that mission on its schedule for launch next year.

    The Kepler Mission

    Looking for planets is necessarily hard work. In the astronomical scheme of things, most planets are very small and Earth-like planets are tremendously, even imperceptibly small. It is difficult enough for astronomers to detect planets on the scale of a Jupiter nearly impossible to find an Earth, some 1,000 times smaller. NASA’s Kepler Mission is the solution to that problem. It’s a space telescope [left] designed to point itself at one field of stars in our galaxy for nearly four years, never wavering from that single point of focus, continuously monitoring the brightness of more than 100,000 stars. The idea behind the mission is to use the transit method of discovery to find extrasolar planets like Earth. A transit occurs when a planet passes between its star and the observer (the Kepler telescope) during which time the star appears momentarily to dim, lasting anywhere from 2 to 16 hours. Of course, the orbit of the planet must be lined up to our plane of view, the chances of which are 0.5 percent for any given sun-like star. But with the tracking of 100,000 stars, NASA hopes at the very least to detect 50 Earth sized planets by the time the mission is complete more if the observable planets prove to be up to twice as large as Earth.

    Life may have emerged not once, but many times on Earth

    IN 4.5 billion years of Earthly history, life as we know it arose just once. Every living thing on our planet shares the same chemistry, and can be traced back to “LUCA”, the last universal common ancestor. So we assume that life must have been really hard to get going, only arising when a nigh-on-impossible set of circumstances combine.

    Or was it? Simple experiments by biologists aiming to recreate life’s earliest moments are challenging that assumption. Life, it seems, is a matter of basic chemistry – no magic required, no rare ingredients, no bolt from the blue.

    And that suggests an even more intriguing possibility. Rather than springing into existence just once in some chemically blessed primordial pond, life may have had many origins. It could have got going over and over again in many different forms for hundreds of thousands of years, only becoming what we see today when everything else was wiped out it in Earth’s first ever mass extinction. In its earliest days on the planet, life as we know it might not have been alone.

    And what about life on other planets? Read about the search for life in the solar system’s other seas

    Just to be clear, what we are talking about came long before animals or plants or even microbes. We are going right back to the start, when the only things fitting the description of “life” were little more than molecular machines. Even then, having stripped away bodies, organs and cells and reduced everything down to the essential reactions, things appear devilishly complex. At a bare minimum, life needs some kind of code, it &hellip

    Subscribe for unlimited digital access

    Subscribe now for unlimited access

    App + Web

    • Unlimited web access
    • New Scientist app
    • Videos of over 200 science talks plus weekly crosswords available exclusively to subscribers
    • Exclusive access to subscriber-only events including our 1st of July Climate Change event
    • A year of unparalleled environmental coverage, exclusively with New Scientist and UNEP

    Print + App + Web

    • Unlimited web access
    • Weekly print edition
    • New Scientist app
    • Videos of over 200 science talks plus weekly crosswords available exclusively to subscribers
    • Exclusive access to subscriber-only events including our 1st of July Climate Change event
    • A year of unparalleled environmental coverage, exclusively with New Scientist and UNEP

    Existing subscribers, please log in with your email address to link your account access.


    Earth life 'may have come from Mars' - History

    Viking 1 - USA Mars Orbiter/Lander - 3,527 kg including fuel - (August 20, 1975 - August 7, 1980)

    • Viking 1 and 2 spacecraft included orbiters (designed after the Mariner 8 and 9 orbiters) and landers. The orbiter weighed 883 kg and the lander 572 kg. Viking 1 was launched from the Kennedy Space Center, on August 20, 1975, the trip to Mars and went into orbit about the planet on June 19, 1976. The lander touched down on July 20, 1976 on the western slopes of Chryse Planitia (Golden Plains). Viking 2 was launched for Mars on November 9, 1975, and landed on September 3, 1976. Both landers had experiments to search for Martian micro-organisms. The results of these experiments are still being debated. The landers provided detailed color panoramic views of the Martian terrain. They also monitored the Martian weather. The orbiters mapped the planet's surface, acquiring over 52,000 images. The Viking project's primary mission ended on November 15, 1976, eleven days before Mars' superior conjunction (its passage behind the Sun), although the Viking spacecraft continued to operate for six years after first reaching Mars. The Viking 1 orbiter was deactivated on August 7, 1980, when it ran out of altitude-control propellant. Viking 1 lander was accidentally shut down on November 13, 1982, and communication was never regained. Its last transmission reached Earth on November 11, 1982. Controllers at NASA's Jet Propulsion Laboratory tried unsuccessfully for another six and one ­half months to regain contact with the lander, but finally closed down the overall mission on 21 May 1983.
      Click here for more information on the Viking missions.
    • Phobos 1 was sent to investigate the Martian moon Phobos. It was lost en route to Mars through a command error on September 2, 1988.
    • Phobos 2 arrived at Mars and was inserted into orbit on January 30, 1989. The orbiter moved within 800 kilometers of Phobos and then failed. The lander never made it to Phobos.
    • Communication was lost with Mars Observer on August 21, 1993, just before it was to be inserted into orbit.
    • Initiated due to the loss of the Mars Observer spacecraft, the Mars Global Surveyor (MGS) mission launched on November 7, 1996. MGS has been in a Martian orbit, successfully mapping the surface since March 1998. Click here to check out the MGS page at JPL.
    • Mars '96 consisted of an orbiter, two landers, and two soil penetrators that were to reach the planet in September 1997. The rocket carrying Mars 96 lifted off successfully, but as it entered orbit the rocket's fourth stage ignited prematurely and sent the probe into a wild tumble. It crashed into the ocean somewhere between the Chilean coast and Easter Island. The spacecraft sank, carrying with it 270 grams of plutonium-238.
    • The Mars Pathfinder delivered a stationary lander and a surface rover to the Red Planet on July 4, 1997. The six-wheel rover, named Sojourner, explored the area near the lander. The mission's primary objective was to demonstrate the feasibility of low-cost landings on the Martian surface. This was the second mission in NASA's low-cost Discovery series. After great scientific success and public interest, the mission formally ended on November 4, 1997, when NASA ended daily communications with the Pathfinder lander and Sojourner rover.
    • Japan's Institute of Space and Astronautical Science (ISAS) launched this probe on July 4, 1998 to study the Martian environment. This would have been the first Japanese spacecraft to reach another planet. The probe was due to arrive at Mars in December of 2003. After revising the flight plan due to earlier problems with the probe, the mission was abandoned on December 9, 2003 when ISAS was unable to communicate with the probe in order to prepare it for orbital insertion.
    • This orbiter was the companion spacecraft to the Mars Surveyor '98 Lander, but the mission failed. Click here to read the Mars Climate Orbiter Mishap Investigation Board's report.
    • The Polar Lander was scheduled to land on Mars on December 3, 1999. Mounted on the cruise stage of the Mars Polar Lander were two Deep Space 2 impact probes, named Amundsen and Scott. The probes had a mass of 3.572 kg each. The cruise stage was to separate from the Mars Polar Lander, and subsequently the two probes were to detach from the cruise stage. The two probes planned to impact the surface 15 to 20 seconds before the Mars Polar Lander was to touch down. Ground crews were unable to contact the spacecraft, and the two probes. NASA concluded that spurious signals during the lander leg deployment caused the spacecraft to think it had landed, resulting in premature shutdown of the spacecraft's engines and destruction of the lander on impact.
    • This Mars orbiter reached the planet on October 24, 2001 and served as a communications relay for future Mars missions. In 2010 Odyssey broke the record for longest-serving spacecraft at the Red Planet. It will support the 2012 landing of the Mars Science Laboratory and surface operations of that mission. Click here for more information.
    • The Mars Express Orbiter and the Beagle 2 lander were launched together on June 2, 2003. The Beagle 2 was released from the Mars Express Orbiter on December 19, 2003. The Mars Express arrived successfully on December 25, 2003. The Beagle 2 was also scheduled to land on December 25, 2003 however, ground controllers have been unable to communicate with the probe. Click here for more information.
    • As part of the Mars Exploration Rover (MER) Mission, "Spirit", also known as MER-A, was launched on June 10, 2003 and successfully arrived on Mars on January 3, 2004. The last communication with Spirit occurred on March 22, 2010. JPL ended attempts to re-establish contact on May 25, 2011. The rover likely lost power due to excessively cold internal temperatures.
    • "Opportunity", also known as MER-B, was launched on July 7, 2003 and successfully arrived on Mars on January 24, 2004. Click here for more information on the MER mission.

    Mars Reconnaissance Orbiter &ndash USA Mars Orbiter - 1,031 kg - (August 12, 2005)

    • The Mars Reconnaissance Orbiter (MRO) was launched on August 12, 2005 for a seven month voyage to Mars. MRO reached Mars in March 10, 2006 and began its scientific mission in November 2006. Click here for more information.

    Phoenix &ndash USA Mars Lander - 350 kg - (August 4, 2007)

    • The Phoenix Mars Lander was launched on August 4, 2007 and landed on Mars on May 25, 2008. It is the first in NASA's Scout Program. Phoenix was designed to study the history of water and habitability potential in the Martian arctic&rsquos ice-rich soil. The solar-powered lander completed its three-month mission and kept working until sunlight waned two months later. The mission was officially ended in May 2010. Click here for more information from the NASA HQ site and here for more from the JPL- University of Arizona site.

    Phobos-Grunt &ndash Russia Mars Lander - 730 kg/Yinghuo-1 &ndash China Mars Orbital Probe &ndash 115 kg - (November 8, 2011)

    • The Phobos-Grunt spacecraft was meant to land on the Martian moon Phobos. The Russian spacecraft did not properly leave Earth&rsquos orbit to set out on its trajectory toward Mars. Yinghuo-1 was a planned Chinese Mars orbital probe launched along with Phobos-Grunt. Both craft were destroyed on re-entry from Earth orbit in January 2012.

    Mars Science Laboratory &ndash USA Mars Rover &ndash 750 kg - (November 26, 2011)

    • The Mars Science Laboratory was launched on November 26, 2011. With its rover named Curiosity, NASA's Mars Science Laboratory mission is designed to assess whether Mars ever had an environment able to support small life forms called microbes. Curiosity landed successfully in Gale Crater at 1:31 am EDT on August 6, 2012. Click here for more information from the NASA JPL site.

    Mars Orbiter Mission (Mangalyaan) &ndash India Mars Orbiter - 15 kg - (November 5, 2013)

    • The Indian Mars Orbiter Mission was launched on November 5, 2013, from the Satish Dhawan Space Center. It was inserted into orbit around Mars on September 24, 2014 and completed its planned 160-day mission duration in March 2015. The spacecraft continues to operate, mapping the planet and measuring radiation.

    MAVEN &ndash USA Mars Orbiter &ndash 2,550 kg - (Launch November 18, 2013)

    • MAVEN (Mars Atmospheric and Volatile EvolutioN) was the second mission selected for NASA's Mars Scout program. It launched on November 18, 2013 and entered orbit around Mars on September 21, 2014. MAVEN&rsquos mission is to obtain critical measurements of the Martian atmosphere to further understanding of the dramatic climate change that has occurred over the course of its history. Click here for more information about MAVEN.

    InSight &ndash USA Mars Lander - (Launch Window March 8 - March 27, 2016)


    An ancient collision

    So, the study authors' group decided to test another theory: What if another planet brought the goodies?

    "Earth could have collided with many different kinds of planets," Grewal told Live Science. Could one of those planets have given the bulk silicate Earth the correct proportion of elements?

    If this collision happened, the two planetary cores would have merged and the two mantles would have merged.

    So, they set out to create a possible planet that could have collided with our own.

    In the lab, in a special kind of furnace, Grewal and his team created the high-temperature, high-pressure conditions under which a planet&rsquos core might form. In capsules of graphite (a form of carbon), they combined metallic powder (which represents the core and includes elements such as iron bound to nitrogen) with different proportions of silicate powder (a mixture of silicon and oxygen, meant to mimic the hypothetical planet&rsquos mantle).

    By varying the temperature, the pressure and the proportions of sulfur in their experiments, the team created scenarios of how these elements could have divided between the core and the rest of the hypothetical planet.

    They found that carbon is much less willing to bond with iron in the presence of high concentrations of nitrogen and sulfur, while nitrogen bonds with iron even when a lot of sulfur is present. So for nitrogen to be excluded from the core, and be present in other parts of the planet, it should have contained very high concentrations of sulfur, Grewal said.

    They then fed these possibilities into a simulation, along with information about how different volatile elements behave, and the present-day amounts of carbon, nitrogen and sulfur in Earth&rsquos outer layers.

    After running over 1 billion simulations, they found that the scenario that made the most sense &mdash the one that had the most probable timing and could lead to a correct ratio of carbon to nitrogen &mdash was one that posited a collision and merger of Earth with a Mars-size planet that contained about 25 to 30 percent sulfur in its core.

    This theory "is very probable," said Célia Dalou, an experimental petrologist at the Centre de Recherches Pétrographiques et Géochimiques in France, who was not a part of the study. "This work is a very successful result of years of research of various different teams."


    Fire and ice

    In 2008, a Mars rover named Phoenix was scooping up soil near the Martian north pole when it found evidence of an unusual salt called perchlorate. This was an exciting find at the time scientists knew that ancient microorganisms on Earth used perchlorate as a source of energy. Perhaps, they thought, this Martian cache of salt served a similar purpose?

    The authors of the new study were excited by the salty discovery for a different reason: Perchlorate is flammable &mdash so flammable it's used on Earth today mainly to make rocket fuel and fireworks burn faster. If perchlorate is abundant in Martian soil, the researchers told NewScientist, then Viking's attempts to heat that soil may have caused the perchlorate to catch fire and instantly obliterate any organic molecules that may have been there.

    The silver lining to this scenario is, if Martian perchlorate did indeed incinerate any carbon-based molecules in Viking's oven, then there would be evidence in the ashes. When carbon burns with perchlorate, it produces a molecule called chlorobenzene &mdash a mix of carbon, hydrogen and chlorine that can last in soil for months. As luck would have it, NASA's Curiosity rover detected traces of chlorobenzene in Martian soil during a 2013 expedition. For further evidence, the researchers decided to go back to Viking itself.

    "We searched the Viking data for a possible reaction product between the salt and organics in the Viking oven," the researchers wrote. The team reanalyzed the original data sets taken during the Viking mission, this time looking specifically for traces of chlorobenzene.

    According to their new paper, the researchers found what they were looking for. The team saw trace amounts of chlorobenzene in samples taken by Viking 2, concluding that the lander may well have held organic matter in the palm of its robotic hand before inadvertently setting the whole lot ablaze.

    Study author Melissa Guzman, a doctoral student at the LATMOS research center in France, told NewScientist that, while this new evidence is compelling, it's not definitive proof of Martian organics. It's possible, for example, that the carbon compounds burned along with the Martian perchlorate in Viking's oven actually originated from Earth and accidentally contaminated the samples.

    Other scientists are ready to believe. Daniel Glavin, a researcher at NASA’s Goddard Space Flight Center in Maryland, who was not involved in the study, told NewScientist that this paper "seals the deal" on Martian organics. Indeed, the study suggests that organic molecules might exist at many sites all over the Red Planet. Whether that means there's microbial life there &mdash and whether humans can confirm that life before setting it ablaze &mdash remains to be seen.


    Contents

    One of the challenges in studying abiogenesis is that the system of reproduction and metabolism utilized by all extant life involves three distinct types of interdependent macromolecules (DNA, RNA, and protein). This suggests that life could not have arisen in its current form, which has led researchers to hypothesize mechanisms whereby the current system might have arisen from a simpler precursor system. The concept of RNA as a primordial molecule [2] can be found in papers by Francis Crick [12] and Leslie Orgel, [13] as well as in Carl Woese's 1967 book The Genetic Code. [14] In 1962, the molecular biologist Alexander Rich posited much the same idea in an article he contributed to a volume issued in honor of Nobel-laureate physiologist Albert Szent-Györgyi. [15] Hans Kuhn in 1972 laid out a possible process by which the modern genetic system might have arisen from a nucleotide-based precursor, and this led Harold White in 1976 to observe that many of the cofactors essential for enzymatic function are either nucleotides or could have been derived from nucleotides. He proposed a scenario whereby the critical electrochemistry of enzymatic reactions would have necessitated retention of the specific nucleotide moieties of the original RNA-based enzymes carrying out the reactions, while the remaining structural elements of the enzymes were gradually replaced by protein, until all that remained of the original RNAs were these nucleotide cofactors, "fossils of nucleic acid enzymes". [16] The phrase "RNA World" was first used by Nobel laureate Walter Gilbert in 1986, in a commentary on how recent observations of the catalytic properties of various forms of RNA fit with this hypothesis. [17]

    The properties of RNA make the idea of the RNA world hypothesis conceptually plausible, though its general acceptance as an explanation for the origin of life requires further evidence. [15] RNA is known to form efficient catalysts and its similarity to DNA makes clear its ability to store information. Opinions differ, however, as to whether RNA constituted the first autonomous self-replicating system or was a derivative of a still-earlier system. [2] One version of the hypothesis is that a different type of nucleic acid, termed pre-RNA, was the first one to emerge as a self-reproducing molecule, to be replaced by RNA only later. On the other hand, the discovery in 2009 that activated pyrimidine ribonucleotides can be synthesized under plausible prebiotic conditions [18] suggests that it is premature to dismiss the RNA-first scenarios. [2] Suggestions for 'simple' pre-RNA nucleic acids have included peptide nucleic acid (PNA), threose nucleic acid (TNA) or glycol nucleic acid (GNA). [19] [20] Despite their structural simplicity and possession of properties comparable with RNA, the chemically plausible generation of "simpler" nucleic acids under prebiotic conditions has yet to be demonstrated. [21]

    RNA as an enzyme Edit

    RNA enzymes, or ribozymes, are found in today's DNA-based life and could be examples of living fossils. Ribozymes play vital roles, such as that of the ribosome. The large subunit of the ribosome includes an rRNA responsible for the peptide bond-forming peptidyl transferase activity of protein synthesis. Many other ribozyme activities exist for example, the hammerhead ribozyme performs self-cleavage [22] and an RNA polymerase ribozyme can synthesize a short RNA strand from a primed RNA template. [23]

    Among the enzymatic properties important for the beginning of life are:

    Self-replication The ability to self-replicate, or synthesize other RNA molecules relatively short RNA molecules that can synthesize others have been artificially produced in the lab. The shortest was 165 bases long, though it has been estimated that only part of the molecule was crucial for this function. One version, 189 bases long, had an error rate of just 1.1% per nucleotide when synthesizing an 11 nucleotide long RNA strand from primed template strands. [24] This 189 base pair ribozyme could polymerize a template of at most 14 nucleotides in length, which is too short for self replication, but is a potential lead for further investigation. The longest primer extension performed by a ribozyme polymerase was 20 bases. [25] In 2016, researchers reported the use of in vitro evolution to improve dramatically the activity and generality of an RNA polymerase ribozyme by selecting variants that can synthesize functional RNA molecules from an RNA template. Each RNA polymerase ribozyme was engineered to remain linked to its new, synthesized RNA strand this allowed the team to isolate successful polymerases. The isolated RNA polymerases were again used for another round of evolution. After several rounds of evolution, they obtained one RNA polymerase ribozyme called 24-3 that was able to copy almost any other RNA, from small catalysts to long RNA-based enzymes. Particular RNAs were amplified up to 10,000 times, a first RNA version of the polymerase chain reaction (PCR). [26] Catalysis The ability to catalyze simple chemical reactions—which would enhance creation of molecules that are building blocks of RNA molecules (i.e., a strand of RNA that would make creating more strands of RNA easier). Relatively short RNA molecules with such abilities have been artificially formed in the lab. [27] [28] A recent study showed that almost any nucleic acid can evolve into a catalytic sequence under appropriate selection. For instance, an arbitrarily chosen 50-nucleotide DNA fragment encoding for the Bos taurus (cattle) albumin mRNA was subjected to test-tube evolution to derive a catalytic DNA (Deoxyribozyme, also called DNAzyme) with RNA-cleavage activity. After only a few weeks, a DNAzyme with significant catalytic activity had evolved. [29] In general, DNA is much more chemically inert than RNA and hence much more resistant to obtaining catalytic properties. If in vitro evolution works for DNA it will happen much more easily with RNA. Amino acid-RNA ligation The ability to conjugate an amino acid to the 3'-end of an RNA in order to use its chemical groups or provide a long-branched aliphatic side-chain. [30] Peptide bond formation The ability to catalyse the formation of peptide bonds between amino acids to produce short peptides or longer proteins. This is done in modern cells by ribosomes, a complex of several RNA molecules known as rRNA together with many proteins. The rRNA molecules are thought responsible for its enzymatic activity, as no amino-acid residues lie within 18Å of the enzyme's active site, [15] and, when the majority of the amino-acid residues in the ribosome were stringently removed, the resulting ribosome retained its full peptidyl transferase activity, fully able to catalyze the formation of peptide bonds between amino acids. [31] A much shorter RNA molecule has been synthesized in the laboratory with the ability to form peptide bonds, and it has been suggested that rRNA has evolved from a similar molecule. [32] It has also been suggested that amino acids may have initially been involved with RNA molecules as cofactors enhancing or diversifying their enzymatic capabilities, before evolving into more complex peptides. Similarly, tRNA is suggested to have evolved from RNA molecules that began to catalyze amino acid transfer. [33]

    RNA in information storage Edit

    RNA is a very similar molecule to DNA, with only two major chemical differences (the backbone of RNA uses ribose instead of deoxyribose and its nucleobases include uracil instead of thymine). The overall structure of RNA and DNA are immensely similar—one strand of DNA and one of RNA can bind to form a double helical structure. This makes the storage of information in RNA possible in a very similar way to the storage of information in DNA. However, RNA is less stable, being more prone to hydrolysis due to the presence of a hydroxyl group at the ribose 2' position.

    Comparison of DNA and RNA structure Edit

    The major difference between RNA and DNA is the presence of a hydroxyl group at the 2'-position of the ribose sugar in RNA (illustration, right). [15] This group makes the molecule less stable because, when not constrained in a double helix, the 2' hydroxyl can chemically attack the adjacent phosphodiester bond to cleave the phosphodiester backbone. The hydroxyl group also forces the ribose into the C3'-endo sugar conformation unlike the C2'-endo conformation of the deoxyribose sugar in DNA. This forces an RNA double helix to change from a B-DNA structure to one more closely resembling A-DNA.

    RNA also uses a different set of bases than DNA—adenine, guanine, cytosine and uracil, instead of adenine, guanine, cytosine and thymine. Chemically, uracil is similar to thymine, differing only by a methyl group, and its production requires less energy. [34] In terms of base pairing, this has no effect. Adenine readily binds uracil or thymine. Uracil is, however, one product of damage to cytosine that makes RNA particularly susceptible to mutations that can replace a GC base pair with a GU (wobble) or AU base pair.

    RNA is thought to have preceded DNA, because of their ordering in the biosynthetic pathways. The deoxyribonucleotides used to make DNA are made from ribonucleotides, the building blocks of RNA, by removing the 2'-hydroxyl group. As a consequence a cell must have the ability to make RNA before it can make DNA.

    Limitations of information storage in RNA Edit

    The chemical properties of RNA make large RNA molecules inherently fragile, and they can easily be broken down into their constituent nucleotides through hydrolysis. [35] [36] These limitations do not make use of RNA as an information storage system impossible, simply energy intensive (to repair or replace damaged RNA molecules) and prone to mutation. While this makes it unsuitable for current 'DNA optimised' life, it may have been acceptable for more primitive life.

    RNA as a regulator Edit

    Riboswitches have been found to act as regulators of gene expression, particularly in bacteria, but also in plants and archaea. Riboswitches alter their secondary structure in response to the binding of a metabolite. This change in structure can result in the formation or disruption of a terminator, truncating or permitting transcription respectively. [37] Alternatively, riboswitches may bind or occlude the Shine–Dalgarno sequence, affecting translation. [38] It has been suggested that these originated in an RNA-based world. [39] In addition, RNA thermometers regulate gene expression in response to temperature changes. [40]

    The RNA world hypothesis is supported by RNA's ability both to store, transmit, and duplicate genetic information, as DNA does, and to perform enzymatic reactions, like protein-based enzymes. Because it can carry out the types of tasks now performed by proteins and DNA, RNA is believed to have once been capable of supporting independent life on its own. [15] Some viruses use RNA as their genetic material, rather than DNA. [41] Further, while nucleotides were not found in experiments based on Miller-Urey experiment, their formation in prebiotically plausible conditions was reported in 2009 [18] a purine base, adenine, is merely a pentamer of hydrogen cyanide. Experiments with basic ribozymes, like Bacteriophage Qβ RNA, have shown that simple self-replicating RNA structures can withstand even strong selective pressures (e.g., opposite-chirality chain terminators). [42]

    Since there were no known chemical pathways for the abiogenic synthesis of nucleotides from pyrimidine nucleobases cytosine and uracil under prebiotic conditions, it is thought by some that nucleic acids did not contain these nucleobases seen in life's nucleic acids. [43] The nucleoside cytosine has a half-life in isolation of 19 days at 100 °C (212 °F) and 17,000 years in freezing water, which some argue is too short on the geologic time scale for accumulation. [44] Others have questioned whether ribose and other backbone sugars could be stable enough to be found in the original genetic material, [45] and have raised the issue that all ribose molecules would have had to be the same enantiomer, as any nucleotide of the wrong chirality acts as a chain terminator. [46]

    Pyrimidine ribonucleosides and their respective nucleotides have been prebiotically synthesised by a sequence of reactions that by-pass free sugars and assemble in a stepwise fashion by including nitrogenous and oxygenous chemistries. In a series of publications, John Sutherland and his team at the School of Chemistry, University of Manchester, have demonstrated high yielding routes to cytidine and uridine ribonucleotides built from small 2- and 3-carbon fragments such as glycolaldehyde, glyceraldehyde or glyceraldehyde-3-phosphate, cyanamide, and cyanoacetylene. One of the steps in this sequence allows the isolation of enantiopure ribose aminooxazoline if the enantiomeric excess of glyceraldehyde is 60% or greater, of possible interest toward biological homochirality. [47] This can be viewed as a prebiotic purification step, where the said compound spontaneously crystallised out from a mixture of the other pentose aminooxazolines. Aminooxazolines can react with cyanoacetylene in a mild and highly efficient manner, controlled by inorganic phosphate, to give the cytidine ribonucleotides. Photoanomerization with UV light allows for inversion about the 1' anomeric centre to give the correct beta stereochemistry one problem with this chemistry is the selective phosphorylation of alpha-cytidine at the 2' position. [48] However, in 2009, they showed that the same simple building blocks allow access, via phosphate controlled nucleobase elaboration, to 2',3'-cyclic pyrimidine nucleotides directly, which are known to be able to polymerise into RNA. [18] Organic chemist Donna Blackmond described this finding as "strong evidence" in favour of the RNA world. [49] However, John Sutherland said that while his team's work suggests that nucleic acids played an early and central role in the origin of life, it did not necessarily support the RNA world hypothesis in the strict sense, which he described as a "restrictive, hypothetical arrangement". [50]

    The Sutherland group's 2009 paper also highlighted the possibility for the photo-sanitization of the pyrimidine-2',3'-cyclic phosphates. [18] A potential weakness of these routes is the generation of enantioenriched glyceraldehyde, or its 3-phosphate derivative (glyceraldehyde prefers to exist as its keto tautomer dihydroxyacetone). [ citation needed ]

    On August 8, 2011, a report, based on NASA studies with meteorites found on Earth, was published suggesting building blocks of RNA (adenine, guanine, and related organic molecules) may have been formed extraterrestrially in outer space. [51] [52] [53] In 2017, a numerical model suggests that the RNA world may have emerged in warm ponds on the early Earth, and that meteorites were a plausible and probable source of the RNA building blocks (ribose and nucleic acids) to these environments. [54] On August 29, 2012, astronomers at Copenhagen University reported the detection of a specific sugar molecule, glycolaldehyde, in a distant star system. The molecule was found around the protostellar binary IRAS 16293-2422, which is located 400 light years from Earth. [55] [56] Because glycolaldehyde is needed to form RNA, this finding suggests that complex organic molecules may form in stellar systems prior to the formation of planets, eventually arriving on young planets early in their formation. [57]

    Nucleotides are the fundamental molecules that combine in series to form RNA. They consist of a nitrogenous base attached to a sugar-phosphate backbone. RNA is made of long stretches of specific nucleotides arranged so that their sequence of bases carries information. The RNA world hypothesis holds that in the primordial soup (or sandwich), there existed free-floating nucleotides. These nucleotides regularly formed bonds with one another, which often broke because the change in energy was so low. However, certain sequences of base pairs have catalytic properties that lower the energy of their chain being created, enabling them to stay together for longer periods of time. As each chain grew longer, it attracted more matching nucleotides faster, causing chains to now form faster than they were breaking down.

    These chains have been proposed by some as the first, primitive forms of life. In an RNA world, different sets of RNA strands would have had different replication outputs, which would have increased or decreased their frequency in the population, i.e. natural selection. As the fittest sets of RNA molecules expanded their numbers, novel catalytic properties added by mutation, which benefitted their persistence and expansion, could accumulate in the population. Such an autocatalytic set of ribozymes, capable of self replication in about an hour, has been identified. It was produced by molecular competition (in vitro evolution) of candidate enzyme mixtures. [58]

    Competition between RNA may have favored the emergence of cooperation between different RNA chains, opening the way for the formation of the first protocell. Eventually, RNA chains developed with catalytic properties that help amino acids bind together (a process called peptide-bonding). These amino acids could then assist with RNA synthesis, giving those RNA chains that could serve as ribozymes the selective advantage. The ability to catalyze one step in protein synthesis, aminoacylation of RNA, has been demonstrated in a short (five-nucleotide) segment of RNA. [59]

    In March 2015, NASA scientists reported that, for the first time, complex DNA and RNA organic compounds of life, including uracil, cytosine, and thymine, have been formed in the laboratory under conditions found only in outer space, using starting chemicals, like pyrimidine, found in meteorites. Pyrimidine, like polycyclic aromatic hydrocarbons (PAHs), may have been formed in red giant stars or in interstellar dust and gas clouds, according to the scientists. [60]

    In 2018, researchers at Georgia Institute of Technology identified three molecular candidates for the bases that might have formed an earliest version of proto-RNA: barbituric acid, melamine, and 2,4,6-triaminopyrimidine (TAP). These three molecules are simpler versions of the four bases in current RNA, which could have been present in larger amounts and could still be forward-compatible with them, but may have been discarded by evolution in exchange for more optimal base pairs. [61] Specifically, TAP can form nucleotides with a large range of sugars. [62] Both TAP and melamine base pair with barbituric acid. All three spontaneously form nucleotides with ribose. [63]

    One of the challenges posed by the RNA world hypothesis is to discover the pathway by which an RNA-based system transitioned to one based on DNA. Geoffrey Diemer and Ken Stedman, at Portland State University in Oregon, may have found a solution. While conducting a survey of viruses in a hot acidic lake in Lassen Volcanic National Park, California, they uncovered evidence that a simple DNA virus had acquired a gene from a completely unrelated RNA-based virus. Virologist Luis Villareal of the University of California Irvine also suggests that viruses capable of converting an RNA-based gene into DNA and then incorporating it into a more complex DNA-based genome might have been common in the Virus world during the RNA to DNA transition some 4 billion years ago. [64] [65] This finding bolsters the argument for the transfer of information from the RNA world to the emerging DNA world before the emergence of the last universal common ancestor. From the research, the diversity of this virus world is still with us.

    Additional evidence supporting the concept of an RNA world has resulted from research on viroids, the first representatives of a novel domain of "subviral pathogens". [66] [67] Viroids are mostly plant pathogens, which consist of short stretches (a few hundred nucleobases) of highly complementary, circular, single-stranded, and non-coding RNA without a protein coat. Compared with other infectious plant pathogens, viroids are extremely small, ranging from 246 to 467 nucleobases. In comparison, the genome of the smallest known viruses capable of causing an infection are about 2,000 nucleobases long. [68]

    In 1989, Diener proposed that, based on their characteristic properties, viroids are more plausible "living relics" of the RNA world than are introns or other RNAs then so considered. [69] If so, viroids have attained potential significance beyond plant pathology to evolutionary biology, by representing the most plausible macromolecules known capable of explaining crucial intermediate steps in the evolution of life from inanimate matter (see: abiogenesis).

    Apparently, Diener's hypothesis lay dormant until 2014, when Flores et al. published a review paper, in which Diener's evidence supporting his hypothesis was summarized. [70] In the same year, a New York Times science writer published a popularized version of Diener's proposal, in which, however, he mistakenly credited Flores et al. with the hypothesis' original conception. [71]

    Pertinent viroid properties listed in 1989 are:

    1. small size, imposed by error-prone replication
    2. high guanine and cytosine content, which increases stability and replication fidelity
    3. circular structure, which assures complete replication without genomic tags
    4. structural periodicity, which permits modular assembly into enlarged genomes
    5. lack of protein-coding ability, consistent with a ribosome-free habitat and
    6. in some cases, replication mediated by ribozymes—the fingerprint of the RNA world. [70]

    The existence, in extant cells, of RNAs with molecular properties predicted for RNAs of the RNA World constitutes an additional argument supporting the RNA World hypothesis.

    Eigen et al. [72] and Woese [73] proposed that the genomes of early protocells were composed of single-stranded RNA, and that individual genes corresponded to separate RNA segments, rather than being linked end-to-end as in present-day DNA genomes. A protocell that was haploid (one copy of each RNA gene) would be vulnerable to damage, since a single lesion in any RNA segment would be potentially lethal to the protocell (e.g. by blocking replication or inhibiting the function of an essential gene).

    Vulnerability to damage could be reduced by maintaining two or more copies of each RNA segment in each protocell, i.e. by maintaining diploidy or polyploidy. Genome redundancy would allow a damaged RNA segment to be replaced by an additional replication of its homolog. However, for such a simple organism, the proportion of available resources tied up in the genetic material would be a large fraction of the total resource budget. Under limited resource conditions, the protocell reproductive rate would likely be inversely related to ploidy number. The protocell's fitness would be reduced by the costs of redundancy. Consequently, coping with damaged RNA genes while minimizing the costs of redundancy would likely have been a fundamental problem for early protocells.

    A cost-benefit analysis was carried out in which the costs of maintaining redundancy were balanced against the costs of genome damage. [74] This analysis led to the conclusion that, under a wide range of circumstances, the selected strategy would be for each protocell to be haploid, but to periodically fuse with another haploid protocell to form a transient diploid. The retention of the haploid state maximizes the growth rate. The periodic fusions permit mutual reactivation of otherwise lethally damaged protocells. If at least one damage-free copy of each RNA gene is present in the transient diploid, viable progeny can be formed. For two, rather than one, viable daughter cells to be produced would require an extra replication of the intact RNA gene homologous to any RNA gene that had been damaged prior to the division of the fused protocell. The cycle of haploid reproduction, with occasional fusion to a transient diploid state, followed by splitting to the haploid state, can be considered to be the sexual cycle in its most primitive form. [74] [75] In the absence of this sexual cycle, haploid protocells with damage in an essential RNA gene would simply die.

    This model for the early sexual cycle is hypothetical, but it is very similar to the known sexual behavior of the segmented RNA viruses, which are among the simplest organisms known. Influenza virus, whose genome consists of 8 physically separated single-stranded RNA segments, [76] is an example of this type of virus. In segmented RNA viruses, "mating" can occur when a host cell is infected by at least two virus particles. If these viruses each contain an RNA segment with a lethal damage, multiple infection can lead to reactivation providing that at least one undamaged copy of each virus gene is present in the infected cell. This phenomenon is known as "multiplicity reactivation". Multiplicity reactivation has been reported to occur in influenza virus infections after induction of RNA damage by UV-irradiation, [77] and ionizing radiation. [78]

    Patrick Forterre has been working on a novel hypothesis, called "three viruses, three domains": [79] that viruses were instrumental in the transition from RNA to DNA and the evolution of Bacteria, Archaea, and Eukaryota. He believes the last universal common ancestor [79] was RNA-based and evolved RNA viruses. Some of the viruses evolved into DNA viruses to protect their genes from attack. Through the process of viral infection into hosts the three domains of life evolved. [79] [80]

    Another interesting proposal is the idea that RNA synthesis might have been driven by temperature gradients, in the process of thermosynthesis. [81] Single nucleotides have been shown to catalyze organic reactions. [82]

    Steven Benner has argued that chemical conditions on the planet Mars, such as the presence of boron, molybdenum, and oxygen, may have been better for initially producing RNA molecules than those on Earth. If so, life-suitable molecules, originating on Mars, may have later migrated to Earth via mechanisms of panspermia or similar process. [83] [84]

    The hypothesized existence of an RNA world does not exclude a "Pre-RNA world", where a metabolic system based on a different nucleic acid is proposed to pre-date RNA. A candidate nucleic acid is peptide nucleic acid (PNA), which uses simple peptide bonds to link nucleobases. [85] PNA is more stable than RNA, but its ability to be generated under prebiological conditions has yet to be demonstrated experimentally.

    Threose nucleic acid (TNA) has also been proposed as a starting point, as has glycol nucleic acid (GNA), and like PNA, also lack experimental evidence for their respective abiogenesis.

    An alternative—or complementary—theory of RNA origin is proposed in the PAH world hypothesis, whereby polycyclic aromatic hydrocarbons (PAHs) mediate the synthesis of RNA molecules. [86] PAHs are the most common and abundant of the known polyatomic molecules in the visible Universe, and are a likely constituent of the primordial sea. [87] PAHs and fullerenes (also implicated in the origin of life) [88] have been detected in nebulae. [89]

    The iron-sulfur world theory proposes that simple metabolic processes developed before genetic materials did, and these energy-producing cycles catalyzed the production of genes.

    Some of the difficulties of producing the precursors on earth are bypassed by another alternative or complementary theory for their origin, panspermia. It discusses the possibility that the earliest life on this planet was carried here from somewhere else in the galaxy, possibly on meteorites similar to the Murchison meteorite. [90] Sugar molecules, including ribose, have been found in meteorites. [91] [92] Panspermia does not invalidate the concept of an RNA world, but posits that this world or its precursors originated not on Earth but rather another, probably older, planet.

    There are hypotheses that are in direct conflict to the RNA world hypothesis [ citation needed ] . The relative chemical complexity of the nucleotide and the unlikelihood of it spontaneously arising, along with the limited number of combinations possible among four base forms, as well as the need for RNA polymers of some length before seeing enzymatic activity, have led some to reject the RNA world hypothesis in favor of a metabolism-first hypothesis, where the chemistry underlying cellular function arose first, along with the ability to replicate and facilitate this metabolism.

    RNA-peptide coevolution Edit

    Another proposal is that the dual-molecule system we see today, where a nucleotide-based molecule is needed to synthesize protein, and a peptide-based (protein) molecule is needed to make nucleic acid polymers, represents the original form of life. [93] This theory is called RNA-peptide coevolution, [94] or the Peptide-RNA world, and offers a possible explanation for the rapid evolution of high-quality replication in RNA (since proteins are catalysts), with the disadvantage of having to postulate the coincident formation of two complex molecules, an enzyme (from peptides) and a RNA (from nucleotides). In this Peptide-RNA World scenario, RNA would have contained the instructions for life, while peptides (simple protein enzymes) would have accelerated key chemical reactions to carry out those instructions. [95] The study leaves open the question of exactly how those primitive systems managed to replicate themselves — something neither the RNA World hypothesis nor the Peptide-RNA World theory can yet explain, unless polymerases (enzymes that rapidly assemble the RNA molecule) played a role. [95]

    A research project completed in March 2015 by the Sutherland group found that a network of reactions beginning with hydrogen cyanide and hydrogen sulfide, in streams of water irradiated by UV light, could produce the chemical components of proteins and lipids, alongside those of RNA. [96] [97] The researchers used the term "cyanosulfidic" to describe this network of reactions. [96] In November 2017, a team at the Scripps Research Institute identified reactions involving the compound diamidophosphate which could have linked the chemical components into short peptide and lipid chains as well as short RNA-like chains of nucleotides. [98] [99]

    The RNA world hypothesis, if true, has important implications for the definition of life. For most of the time that followed Franklin, Watson and Crick's elucidation of DNA structure in 1953, life was largely defined in terms of DNA and proteins: DNA and proteins seemed the dominant macromolecules in the living cell, with RNA only aiding in creating proteins from the DNA blueprint.

    The RNA world hypothesis places RNA at center-stage when life originated. The RNA world hypothesis is supported by the observations that ribosomes are ribozymes: [100] [101] the catalytic site is composed of RNA, and proteins hold no major structural role and are of peripheral functional importance. This was confirmed with the deciphering of the 3-dimensional structure of the ribosome in 2001. Specifically, peptide bond formation, the reaction that binds amino acids together into proteins, is now known to be catalyzed by an adenine residue in the rRNA.

    RNAs are known to play roles in other cellular catalytic processes, specifically in the targeting of enzymes to specific RNA sequences. In eukaryotes, the processing of pre-mRNA and RNA editing take place at sites determined by the base pairing between the target RNA and RNA constituents of small nuclear ribonucleoproteins (snRNPs). Such enzyme targeting is also responsible for gene down regulation through RNA interference (RNAi), where an enzyme-associated guide RNA targets specific mRNA for selective destruction. Likewise, in eukaryotes the maintenance of telomeres involves copying of an RNA template that is a constituent part of the telomerase ribonucleoprotein enzyme. Another cellular organelle, the vault, includes a ribonucleoprotein component, although the function of this organelle remains to be elucidated.


    Who is Boriska Kipriyanovich?

    Born in 1996, Boris Kipriyanovich, who goes by the name Boriska which means “little Boris”, is considered a child genius.

    His mother is a doctor and says she knew he was special as soon as he held his head up without any support just two weeks after being born.

    She claims he started speaking a few months later and by the age of one and a half was able to read, draw and paint.

    While Boriska was going to kindergarten at the age of just two, his teachers couldn’t help but notice his incredible writing and language talents along with his astonishing memory skills.

    The boy has repeatedly claimed that he was previously a martian pilot who travelled to Earth.

    Boriska’s mother and father claim they didn’t teach their son anything about space as a child but say he would often sit and talk about Mars, the planetary systems and alien civilisations.

    They say his fascination with space soon became his number one interest - and it wasn't long before he started claiming to have been born on Mars.

    Researchers have described him as an extremely shy young man with above-average intelligence.

    His outstanding knowledge of the planetary systems has confounded experts around the world, including scientists.


    The secret of how life on Earth began

    Today life has conquered every square inch of Earth, but when the planet formed it was a dead rock. How did life get started?

    This story is part of BBC Earth's "Best of 2016" list, our greatest hits of the year. Browse the full list.

    How did life begin? There can hardly be a bigger question. For much of human history, almost everyone believed some version of "the gods did it". Any other explanation was inconceivable.

    That is no longer true. Over the last century, a few scientists have tried to figure out how the first life might have sprung up. They have even tried to recreate this Genesis moment in their labs: to create brand-new life from scratch.

    So far nobody has managed it, but we have come a long way. Today, many of the scientists studying the origin of life are confident that they are on the right track &ndash and they have the experiments to back up their confidence.

    This is the story of our quest to discover our ultimate origin. It is a story of obsession, struggle and brilliant creativity, which encompasses some of the greatest discoveries of modern science. The endeavour to understand life's beginnings has sent men and women to the furthest corners of our planet. Some of the scientists involved have been bedevilled as monsters, while others had to do their work under the heel of brutal totalitarian governments.

    This is the story of the birth of life on Earth.

    Life is old. The dinosaurs are perhaps the most famous extinct creatures, and they had their beginnings 250 million years ago. But life dates back much further.

    The oldest known fossils are around 3.5 billion years old, 14 times the age of the oldest dinosaurs. But the fossil record may stretch back still further. For instance, in August 2016 researchers found what appear to be fossilised microbes dating back 3.7 billion years.

    The Earth itself is not much older, having formed 4.5 billion years ago.

    If we assume that life formed on Earth &ndash which seems reasonable, given that we have not yet found it anywhere else &ndash then it must have done so in the billion years between Earth coming into being and the preservation of the oldest known fossils.

    As well as narrowing down when life began, we can make an educated guess at what it was.

    Since the 19th Century, biologists have known that all living things are made of "cells": tiny bags of living matter that come in different shapes and sizes. Cells were first discovered in the 17th Century, when the first modern microscopes were invented, but it took well over a century for anyone to realise that they were the basis of all life.

    Using only the materials and conditions found on the Earth over 3.5 billion years ago, we have to make a cell

    You might not think you look much like a catfish or a Tyrannosaurus rex, but a microscope will reveal that you are all made of pretty similar kinds of cells. So are plants and fungi.

    But by far the most numerous forms of life are microorganisms, each of which is made up of just one cell. Bacteria are the most famous group, and they are found everywhere on Earth.

    In April 2016, scientists presented an updated version of the "tree of life": a kind of family tree for every living species. Almost all of the branches are bacteria. What's more, the shape of the tree suggests that a bacterium was the common ancestor of all life. In other words, every living thing &ndash including you &ndash is ultimately descended from a bacterium.

    This means we can define the problem of the origin of life more precisely. Using only the materials and conditions found on the Earth over 3.5 billion years ago, we have to make a cell.

    Chapter 1. The first experiments

    For most of history, it was not really considered necessary to ask how life began, because the answer seemed obvious.

    Before the 1800s, most people believed in "vitalism". This is the intuitive idea that living things were endowed with a special, magical property that made them different from inanimate objects.

    The chemicals of life can all be made from simpler chemicals that have nothing to do with life

    Vitalism was often bound up with cherished religious beliefs. The Bible says that God used "the breath of life" to animate the first humans, and the immortal soul is a form of vitalism.

    There is just one problem. Vitalism is plain wrong.

    By the early 1800s, scientists had discovered several substances that seemed to be unique to life. One such chemical was urea, which is found in urine and was isolated in 1799.

    This was still, just, compatible with vitalism. Only living things seemed to be able to make these chemicals, so perhaps they were infused with life energy and that was what made them special.

    But in 1828, the German chemist Friedrich Wöhler found a way to make urea from a common chemical called ammonium cyanate, which had no obvious connection with living things. Others followed in his footsteps, and it was soon clear that the chemicals of life can all be made from simpler chemicals that have nothing to do with life.

    This was the end of vitalism as a scientific concept. But people found it profoundly hard to let go of the idea. For many, saying that there is nothing "special" about the chemicals of life seemed to rob life of its magic, to reduce us to mere machines. It also, of course, contradicted the Bible.

    The mystery of life's origin was ignored for decades

    Even scientists have struggled to shed vitalism. As late as 1913, the English biochemist Benjamin Moore was fervently pushing a theory of "biotic energy", which was essentially vitalism under a different name. The idea had a strong emotional hold.

    Today the idea clings on in unexpected places. For example, there are plenty of science-fiction stories in which a person's "life energy" can be boosted or drained away. Think of the "regeneration energy" used by the Time Lords in Doctor Who, which can even be topped up if it runs low. This feels futuristic, but it is a deeply old-fashioned idea.

    Still, after 1828 scientists had legitimate reasons to look for a deity-free explanation for how the first life formed. But they did not. It seems like an obvious subject to explore, but in fact the mystery of life's origin was ignored for decades. Perhaps everyone was still too emotionally attached to vitalism to take the next step.

    Instead, the big biological breakthrough of the 19th Century was the theory of evolution, as developed by Charles Darwin and others.

    Darwin knew that it was a profound question

    Darwin's theory, set out in On the Origin of Species in 1859, explained how the vast diversity of life could all have arisen from a single common ancestor. Instead of each of the different species being created individually by God, they were all descended from a primordial organism that lived millions of years ago: the last universal common ancestor.

    This idea proved immensely controversial, again because it contradicted the Bible. Darwin and his ideas came under ferocious attack, particularly from outraged Christians.

    The theory of evolution said nothing about how that first organism came into being.

    Darwin knew that it was a profound question, but &ndash perhaps wary of starting yet another fight with the Church &ndash he only seems to have discussed the issue in a letter written in 1871. His excitable language reveals that he knew the deep significance of the question:

    The first hypothesis for the origin of life was invented in a savagely totalitarian country

    "But if (& oh what a big if) we could conceive in some warm little pond with all sorts of ammonia & phosphoric salts,&mdashlight, heat, electricity &c present, that a protein compound was chemically formed, ready to undergo still more complex changes. "

    In other words, what if there was once a small body of water, filled with simple organic compounds and bathed in sunlight. Some of those compounds might combine to form a life-like substance such as a protein, which could then start evolving and becoming more complex.

    It was a sketchy idea. But it would become the basis of the first hypothesis for how life began.

    This idea emerged from an unexpected place. You might think that this daring piece of free thinking would have been developed in a democratic country with a tradition of free speech: perhaps the United States. But in fact the first hypothesis for the origin of life was invented in a savagely totalitarian country, where free thinking was stamped out: the USSR.

    In Stalin's Russia, everything was under the control of the state. That included people's ideas, even on subjects &ndash like biology &ndash that seem unrelated to Communist politics.

    Oparin imagined what Earth was like when it was newly formed

    Most famously, Stalin effectively banned scientists from studying conventional genetics. Instead he imposed the ideas of a farm worker named Trofim Lysenko, which he thought were more in line with Communist ideology. Scientists working on genetics were forced to publicly support Lysenko's ideas, or risk ending up in a labour camp.

    It was in this repressive environment that Alexander Oparin carried out his research into biochemistry. He was able to keep working because he was a loyal Communist: he supported Lysenko's ideas and even received the Order of Lenin, the highest decoration that could be bestowed on someone living in the USSR.

    In 1924, Oparin published his book The Origin of Life. In it he set out a vision for the birth of life that was startlingly similar to Darwin's warm little pond.

    Oparin imagined what Earth was like when it was newly formed. The surface was searingly hot, as rocks from space plunged down onto it and impacted. It was a mess of semi-molten rocks, containing a huge range of chemicals &ndash including many based on carbon.

    If you watch coacervates under a microscope, they behave unnervingly like living cells

    Eventually the Earth cooled enough for water vapour to condense into liquid water, and the first rain fell. Before long Earth had oceans, which were hot and rich in carbon-based chemicals. Now two things could happen.

    First, the various chemicals could react with each other to form lots of new compounds, some of which would be more complex. Oparin supposed that the molecules central to life, like sugars and amino acids, could all have formed in Earth's waters.

    Second, some of the chemicals began to form microscopic structures. Many organic chemicals do not dissolve in water: for example, oil forms a layer on top of water. But when some of these chemicals contact water they form spherical globules called "coacervates", which can be up to 0.01cm (0.004 inches) across.

    If you watch coacervates under a microscope, they behave unnervingly like living cells. They grow and change shape, and sometimes divide into two. They can also take in chemicals from the surrounding water, so life-like chemicals can become concentrated inside them. Oparin proposed that coacervates were the ancestors of modern cells.

    The idea that living organisms formed by purely chemical means, without a god or even a "life force", was radical

    Five years later in 1929, the English biologist J. B. S. Haldane independently proposed some very similar ideas in a short article published in the Rationalist Annual.

    Haldane had already made enormous contributions to evolutionary theory, helping to integrate Darwin's ideas with the emerging science of genetics.

    He was also a larger-than-life character. On one occasion, he suffered a perforated eardrum thanks to some experiments with decompression chambers, but later wrote that: "the drum generally heals up and if a hole remains in it, although one is somewhat deaf, one can blow tobacco smoke out of the ear in question, which is a social accomplishment."

    Just like Oparin, Haldane outlined how organic chemicals could build up in water, "[until] the primitive oceans reached the consistency of hot dilute soup". This set the stage for "the first living or half-living things" to form, and for each one to become enclosed in "an oily film".

    It is telling that of all the biologists in the world, it was Oparin and Haldane who proposed this. The idea that living organisms formed by purely chemical means, without a god or even a "life force", was radical. Like Darwin's theory of evolution before it, it flew in the face of Christianity.

    There was one problem. There was no experimental evidence to back it up

    That suited the USSR just fine. The Soviet regime was officially atheist, and its leaders were eager to support materialistic explanations for profound phenomena like life. Haldane was also an atheist, and a devoted communist to boot.

    "At that time, to accept or not accept this idea depended essentially on personalities: whether they were religious or whether they supported left or communist ideas," says origin-of-life expert Armen Mulkidjanian of the University of Osnabrück in Germany. "In the Soviet Union they were accepted happily because they didn't need God. In the western world, if you look for people who were thinking in this direction, they all were lefties, communists and so on."

    The idea that life formed in a primordial soup of organic chemicals became known as the Oparin-Haldane hypothesis. It was neat and compelling, but there was one problem. There was no experimental evidence to back it up. This would not arrive for almost a quarter of a century.

    By the time Harold Urey became interested in the origin of life, he had already won the 1934 Nobel Prize in Chemistry and helped to build the atomic bomb. During World War Two Urey worked on the Manhattan Project, collecting the unstable uranium-235 needed for the bomb's core. After the war he fought to keep nuclear technology in civilian control.

    In 1952, Miller began the most famous experiment on the origin of life ever attempted

    He also became interested in the chemistry of outer space, particularly what went on when the Solar System was first forming. One day he gave a lecture and pointed out that there was probably no oxygen in Earth's atmosphere when it first formed. This would have offered the ideal conditions for Oparin and Haldane's primordial soup to form: the fragile chemicals would have been destroyed by contact with oxygen.

    A doctoral student named Stanley Miller was in the audience, and later approached Urey with a proposal: could they test this idea? Urey was sceptical, but Miller talked him into it.

    So in 1952, Miller began the most famous experiment on the origin of life ever attempted.

    The set-up was simple. Miller connected a series of glass flasks and circulated four chemicals that he suspected were present on the early Earth: boiling water, hydrogen gas, ammonia and methane. He subjected the gases to repeated electric shocks, to simulate the lightning strikes that would have been a common occurrence on Earth so long ago.

    You can go from a simple atmosphere and produce lots of biological molecules

    Miller found that "the water in the flask became noticeably pink after the first day, and by the end of the week the solution was deep red and turbid". Clearly, a mix of chemicals had formed.

    When Miller analysed the mixture he found that it contained two amino acids: glycine and alanine. Amino acids are often described as the building blocks of life. They are used to form the proteins that control most biochemical processes in our bodies. Miller had made two of life's most important components, from scratch.

    The results were published in the prestigious journal Science in 1953. Urey, in a selfless act unusual among senior scientists, had his name taken off the paper, giving Miller sole credit. Despite this, the study is often known as the "Miller-Urey experiment".

    "The strength of Miller-Urey is to show that you can go from a simple atmosphere and produce lots of biological molecules," says John Sutherland of the Laboratory of Molecular Biology in Cambridge, UK.

    Life was more complicated than anyone had thought

    The details turned out to be wrong, since later studies showed that the early Earth's atmosphere had a different mix of gases. But that is almost beside the point.

    "It was massively iconic, stimulated the public's imagination and continues to be cited extensively," says Sutherland.

    In the wake of Miller's experiment, other scientists began finding ways to make simple biological molecules from scratch. A solution to the mystery of the origin of life seemed close.

    But then it became clear that life was more complicated than anyone had thought. Living cells, it turned out, were not just bags of chemicals: they were intricate little machines. Suddenly, making one from scratch began to look like a much bigger challenge than scientists had anticipated.

    Chapter 2. The great polarisation

    By the early 1950s, scientists had moved away from the long-standing assumption that life was a gift from the gods. They had instead begun to explore the possibility that life formed spontaneously and naturally on the early Earth &ndash and thanks to Stanley Miller's iconic experiment, they even had some practical support for the idea.

    While Miller was trying to make the stuff of life from scratch, other scientists were figuring out what genes were made of.

    By this time, many biological molecules were known. These included sugars, fats, proteins &ndash and nucleic acids such as "deoxyribonucleic acid", or DNA for short.

    Theirs was one of the greatest scientific discoveries of the 20th Century

    Today we take it for granted that DNA carries our genes, but this actually came as a shock to 1950s biologists. Proteins are more complex, so scientists thought they were the genes.

    That idea was disproved in 1952 by Alfred Hershey and Martha Chase of the Carnegie Institution of Washington. They studied simple viruses that only contain DNA and protein, and which have to infect bacteria in order to reproduce. They found that it was the viral DNA that entered the bacteria: the proteins stayed outside. Clearly, DNA was the genetic material.

    Hershey and Chase's findings triggered a frantic race to figure out the structure of DNA, and thus how it worked. The following year, the problem was cracked by Francis Crick and James Watson of the University of Cambridge, UK &ndash with a lot of under-acknowledged help from their colleague Rosalind Franklin.

    Theirs was one of the greatest scientific discoveries of the 20th Century. It also reshaped the search for the origin of life, by revealing the incredible intricacy that is hidden inside living cells.

    Crick and Watson realised that DNA is a double helix, like a ladder that has been twisted into a spiral. The two "poles" of the ladder are each built from molecules called nucleotides.

    Your genes ultimately come from an ancestral bacterium

    This structure explained how cells copy their DNA. In other words, it revealed how parents make copies of their genes and pass them on to their children.

    The key point is that the double helix can be "unzipped". This exposes the genetic code &ndash made up of sequences of the genetic bases A, T, C and G &ndash that is normally locked away inside the DNA ladder&rsquos "rungs". Each strand is then used as a template to recreate a copy of the other.

    Using this mechanism, genes have been passed down from parent to child since the beginning of life. Your genes ultimately come from an ancestral bacterium &ndash and at every step they were copied using the mechanism Crick and Watson discovered.

    Explore the structure of DNA in this video:

    Crick and Watson set out their findings in a 1953 paper in Nature. Over the next few years, biochemists raced to figure out exactly what information DNA carries, and how that information is used in living cells. The innermost secrets of life were being exposed for the first time.

    Suddenly, Oparin and Haldane's ideas looked naively simple

    It turned out that DNA only has one job. Your DNA tells your cells how to make proteins: molecules that perform a host of essential tasks. Without proteins you could not digest your food, your heart would stop and you could not breathe.

    But the process of using DNA to make proteins proved to be staggeringly intricate. That was a big problem for anyone trying to explain the origin of life, because it is hard to imagine how something so complex could ever have got started.

    Each protein is essentially a long chain of amino acids, strung together in a specific order. The sequence of the amino acids determines the three-dimensional shape of the protein, and thus what it does.

    That information is encoded in the sequence of the DNA's bases. So when a cell needs to make a particular protein, it reads the relevant gene in the DNA to get the sequence of amino acids.

    It turned out that DNA only has one job

    But there is a twist. DNA is precious, so cells prefer to keep it bundled away safely. For this reason, they copy the information from DNA onto short molecules of another substance called RNA (ribonucleic acid). If DNA is a library book, RNA is a scrap of paper with a key passage scribbled onto it. RNA is similar to DNA, except that it only has one strand.

    Finally, the process of converting the information in that RNA strand into a protein takes place in an enormously elaborate molecule called a "ribosome".

    This process is going on in every living cell, even the simplest bacteria. It is as essential to life as eating and breathing. Any explanation for the origin of life must show how this complex trinity &ndash DNA, RNA and ribosome protein &ndash came into existence and started working.

    Suddenly, Oparin and Haldane's ideas looked naively simple, while Miller's experiment, which only produced a few of the amino acids used to build proteins, looked amateurish. Far from taking us most of the way to creating life, his seminal study was clearly just the first step on a long road.

    The idea that life began with RNA would prove enormously influential

    "DNA makes RNA makes protein, all in this lipid-encapsulated bag of chemicals," says John Sutherland. "You look at that and it's just 'wow, that's too complicated'. How are we going to find organic chemistry that will make all that in one go?"

    The first person to really tackle this head-on was a British chemist named Leslie Orgel. He was one of the first to see Crick and Watson's model of DNA, and would later help Nasa with their Viking programme, which sent robotic landers to Mars.

    Orgel set out to simplify the problem. Writing in 1968, and supported by Crick, he suggested that the first life did not have proteins or DNA. Instead, it was made almost entirely of RNA. For this to work, these primordial RNA molecules must have been particularly versatile. For one thing, they must have been able to build copies of themselves, presumably using the same base-pairing mechanism as DNA.

    The idea that life began with RNA would prove enormously influential. But it also triggered a scientific turf war that has lasted until the present day.

    By suggesting that life began with RNA and little else, Orgel was proposing that one crucial aspect of life &ndash its ability to reproduce itself &ndash appeared before all the others. In a sense, he was not just suggesting how life was first assembled: he was saying something about what life is.

    Scientists studying the origin of life split into camps

    Many biologists would agree with Orgel's "replication first" idea. In Darwin's theory of evolution, the ability to create offspring is absolutely central: the only way an organism can "win" is to leave behind lots of children.

    But there are other features of life that seem equally essential. The most obvious is metabolism: the ability to extract energy from your surroundings and use it to keep yourself alive. For many biologists, metabolism must have been the original defining feature of life, with replication emerging later.

    So from the 1960s onwards, scientists studying the origin of life split into camps.

    "The basic polarisation was metabolism-first versus genetics-first," says Sutherland.

    Scientific meetings on the origin of life have often been fractious affairs

    Meanwhile, a third group maintained that the first thing to appear was a container for the key molecules, to keep them from floating off. "Compartmentalisation must have come first, because there's no point doing metabolism unless you're compartmentalised," says Sutherland. In other words, there needed to be a cell &ndash as Oparin and Haldane had emphasised a few decades earlier &ndash perhaps enclosed by a membrane of simple fats and lipids.

    All three ideas acquired adherents and have survived to the present day. Scientists have become passionately committed to their pet ideas, sometimes blindly so.

    As a result, scientific meetings on the origin of life have often been fractious affairs, and journalists covering the subject are regularly told by a scientist in one camp that the ideas emerging from the other camps are stupid or worse.

    Thanks to Orgel, the idea that life began with RNA and genetics got off to an early head start. Then came the 1980s, and a startling discovery that seemed to pretty much confirm it.

    Chapter 3. Search for the first replicator

    After the 1960s, the scientists on the quest to understand life's origins split into three groups. Some were convinced that life began with the formation of primitive versions of biological cells. Others thought the key first step was a metabolic system, and yet others focused on the importance of genetics and replication. This last group began trying to figure out what that first replicator might have looked like &ndash with a focus on the idea that it was made of RNA.

    As early as the 1960s, scientists had reason to think RNA was the source of all life.

    Specifically, RNA can do something that DNA cannot. It is a single-stranded molecule, so unlike stiff, double-stranded DNA it can fold itself into a range of different shapes.

    You could not live without enzymes

    RNA's origami-like folding looked rather similar to the way proteins behave. Proteins are also basically long strands &ndash made of amino acids rather than nucleotides &ndash and this allows them to construct elaborate structures.

    This is the key to proteins' most amazing ability. Some of them can speed up, or "catalyse", chemical reactions. These proteins are known as enzymes.

    Many enzymes are found in your guts, where they break up the complex molecules from your food into simple ones like sugars that your cells can use. You could not live without enzymes.

    Leslie Orgel and Francis Crick had a suspicion. If RNA could fold like a protein, maybe it could form enzymes. If that were true, RNA could have been the original &ndash and highly versatile &ndash living molecule, storing information as DNA does now and catalysing reactions as some proteins do.

    It was a neat idea, but there would be no proof for over a decade.

    Thomas Cech was born and raised in Iowa. As a child he was fascinated by rocks and minerals. By the time he was in junior high school he was visiting the local university and knocking on geologists' doors, asking to see models of mineral structures.

    But he eventually wound up becoming a biochemist, focusing on RNA.

    Now the notion that life began with RNA was looking promising

    In the early 1980s, Cech and his colleagues at the University of Colorado Boulder were studying a single-celled organism called Tetrahymena thermophila. Part of its cellular machinery includes strands of RNA. Cech found that one particular section of the RNA sometimes detached from the rest, as if something had cut it out with scissors.

    When the team removed all the enzymes and other molecules that might be acting as molecular scissors, the RNA kept doing it. They had discovered the first RNA enzyme: a short piece of RNA that was able to cut itself out of the larger strand it was part of.

    Cech published the results in 1982. The following year, another group found a second RNA enzyme &ndash or "ribozyme", as it was dubbed.

    Finding two RNA enzymes in quick succession suggested that there were plenty more out there. Now the notion that life began with RNA was looking promising.

    Discover more about RNA in this video:

    It would be Walter Gilbert of Harvard University in Cambridge, Massachusetts who gave the idea a name. A physicist who had become fascinated by molecular biology, Gilbert would also be one of the early advocates of sequencing the human genome.

    The RNA World is an elegant way to make complex life from scratch

    Writing in Nature in 1986, Gilbert proposed that life began in the "RNA World".

    The first stage of evolution, Gilbert argued, consisted of "RNA molecules performing the catalytic activities necessary to assemble themselves from a nucleotide soup". By cutting and pasting different bits of RNA together, the RNA molecules could create ever more useful sequences. Eventually they found a way to make proteins and protein enzymes, which proved so useful that they largely supplanted the RNA versions and gave rise to life as we recognise it today.

    The RNA World is an elegant way to make complex life from scratch. Instead of having to rely on the simultaneous formation of dozens of biological molecules from the primordial soup, one Jack-of-all-trades molecule could do the work of all of them.

    In 2000, the RNA World hypothesis was gifted a dramatic piece of supporting evidence.

    Thomas Steitz had spent 30 years studying the structures of the molecules in living cells. In the 1990s he took on his biggest challenge: figuring out the structure of the ribosome.

    The fact that this essential machine was based on RNA made the RNA World even more plausible

    Every living cell has a ribosome. This huge molecule reads instructions from RNA and strings together amino acids to make proteins. The ribosomes in your cells built most of your body.

    The ribosome was known to contain RNA. But in 2000 Steitz's team produced a detailed image of the ribosome's structure, which showed that the RNA was the catalytic core of the ribosome.

    This was critical, because the ribosome is so fundamental to life, and so ancient. The fact that this essential machine was based on RNA made the RNA World even more plausible.

    RNA World supporters were ecstatic at the discovery, and in 2009 Steitz would receive a share of a Nobel Prize. But since then, doubts have crept back in.

    Right from the start, there were two problems with the RNA World idea. Could RNA really perform all the functions of life by itself? And could it have formed on the early Earth?

    They set out to make a self-replicating RNA for themselves

    It is 30 years since Gilbert set out the stall for the RNA World, and we still do not have hard evidence that RNA can do all the things the theory demands of it. It is a handy little molecule, but it may not be handy enough.

    One task stood out. If life began with an RNA molecule, that RNA must have been able to make copies of itself: it should have been self-replicating.

    But no known RNA can self-replicate. Nor can DNA. It takes a battalion of enzymes and other molecules to build a replica copy of a piece of RNA or DNA.

    So in the late 1980s, a few biologists started a rather quixotic quest. They set out to make a self-replicating RNA for themselves.

    Jack Szostak of the Harvard Medical School was one of the first to get involved. As a child he was so fascinated with chemistry that he had a lab in his basement. With a splendid disregard for his own safety, he once set off an explosion that embedded a glass tube into the ceiling.

    They had showed that RNA enzymes could be truly powerful

    In the early 1980s, Szostak helped to show how our genes protect themselves against the ageing process. This early research would eventually net him a share of a Nobel Prize.

    But he soon became fascinated by Cech's RNA enzymes. "I thought that work was just really cool," he says. "In principle, there might be a possibility for RNA to catalyse its own replication."

    In 1988, Cech found an RNA enzyme that could build a short RNA molecule about 10 nucleotides long. Szostak set out to improve on the discovery by evolving new RNA enzymes in the lab. His team created a pool of random sequences and tested them to see which ones showed catalytic activity. They then took those sequences, tweaked them, and tested again.

    After 10 rounds of this, Szostak had produced an RNA enzyme that made a reaction go seven million times faster than it naturally would. They had showed that RNA enzymes could be truly powerful. But their enzyme could not copy itself, not even close. Szostak had hit a wall.

    The next big advance came in 2001 from Szostak's former student David Bartel, of the Massachusetts Institute of Technology in Cambridge. Bartel made an RNA enzyme called R18 that could add new nucleotides to a strand of RNA, based on an existing template. In other words, it was not just adding random nucleotides: it was correctly copying a sequence.

    This was still not a self-replicator, but it was edging towards it. R18 consisted of a string of 189 nucleotides, and it could reliably add 11 nucleotides to a strand: 6% of its own length. The hope was that a few tweaks would allow it to make a strand 189 nucleotides long &ndash as long as itself.

    RNA does not seem to be up to the job of kick-starting life

    The best attempt came in 2011 from Philipp Holliger of the Laboratory of Molecular Biology in Cambridge, UK. His team created a modified R18 called tC19Z, which copies sequences up to 95 nucleotides long. That is 48% of its own length: more than R18, but not the necessary 100%.

    An alternative approach has been put forward by Gerald Joyce and Tracey Lincoln of the Scripps Research Institute in La Jolla, California. In 2009 they created an RNA enzyme that replicates itself indirectly.

    Their enzyme joins together two short pieces of RNA to create a second enzyme. This then joins together another two RNA pieces to recreate the original enzyme.

    This simple cycle could be continued indefinitely, given the raw materials. But the enzymes only worked if they were given the correct RNA strands, which Joyce and Lincoln had to make.

    For the many scientists who are sceptical about the RNA World, the lack of a self-replicating RNA is a fatal problem with the idea. RNA does not seem to be up to the job of kick-starting life.

    Maybe there was some other type of molecule on the early Earth

    The case has also been weakened by chemists' failure to make RNA from scratch. It looks like a simple molecule compared to DNA, but RNA has proved to be enormously difficult to make.

    The problem is the sugar and the base that make up each nucleotide. It is possible to make each of them individually, but the two stubbornly refuse to link together.

    This problem was already clear by the early 1990s. It left many biologists with a nagging suspicion that the RNA World hypothesis, while neat, could not be quite right.

    Instead, maybe there was some other type of molecule on the early Earth: something simpler than RNA, which really could assemble itself out of the primordial soup and start self-replicating. This might have come first, and then led to RNA, DNA and the rest.

    In 1991, Peter Nielsen of the University of Copenhagen in Denmark came up with a candidate for the primordial replicator.

    It was essentially a heavily-modified version of DNA. Nielsen kept the bases the same &ndash sticking with the A, T, C and G found in DNA &ndash but made the backbone out of molecules called polyamides instead of the sugars found in DNA. He called the new molecule polyamide nucleic acid, or PNA. Confusingly, it has since become known as peptide nucleic acid.

    PNA, unlike RNA, might have formed readily on the early Earth

    PNA has never been found in nature. But it behaves a lot like DNA. A strand of PNA can even take the place of one of the strands in a DNA molecule, with the complementary bases pairing up as normal. What's more, PNA can coil up into a double helix, just like DNA.

    Stanley Miller was intrigued. Deeply sceptical about the RNA World, he suspected that PNA was a more plausible candidate for the first genetic material.

    In 2000 he produced some hard evidence. By then he was 70 years old, and had just suffered the first in a series of debilitating strokes that would ultimately leave him confined to a nursing home, but he was not quite done. He repeated his classic experiment, which we discussed in Chapter One, this time using methane, nitrogen, ammonia and water &ndash and obtained the polyamide backbone of PNA.

    This suggested that PNA, unlike RNA, might have formed readily on the early Earth.

    Other chemists have come up with their own alternative nucleic acids.

    Each of these alternative nucleic acids has its supporters: usually, the person who made it

    In 2000, Albert Eschenmoser made threose nucleic acid (TNA). This is basically DNA, but with a different sugar in its backbone. Strands of TNA can pair up to form a double helix, and information can be copied back and forth between RNA and TNA.

    What's more, TNA can fold up into complex shapes, and even bind to a protein. This hints that TNA could act as an enzyme, just like RNA.

    Each of these alternative nucleic acids has its supporters: usually, the person who made it. But there is no trace of them in nature, so if the first life did use them, at some point it must have utterly abandoned them in favour of RNA and DNA. This might be true, but there is no evidence.

    All this meant that, by the mid-2000s, supporters of the RNA World were in a quandary.

    The RNA World, neat as it was, could not be the whole truth

    On the one hand, RNA enzymes existed and they included one of the most important pieces of biological machinery, the ribosome. That was good.

    But no self-replicating RNA had been found, and nobody could figure out how RNA formed in the primordial soup. The alternative nucleic acids might solve the latter problem, but there was no evidence they ever existed in nature. That was less good.

    The obvious conclusion was that the RNA World, neat as it was, could not be the whole truth.

    Meanwhile, a rival theory had been steadily gathering steam since the 1980s. Its supporters argue that life did not begin with RNA, or DNA, or any other genetic substance. Instead it began as a mechanism for harnessing energy.

    Chapter 4. Power from protons

    We saw in Chapter Two how scientists divided into three schools of thought about how life began. One group was convinced that life began with a molecule of RNA, but they struggled to work out how RNA or similar molecules could have formed spontaneously on the early Earth and then made copies of themselves. Their efforts were exciting at first, but ultimately frustrating. However, even while this research was progressing, there were other origin-of-life researchers who felt sure that life began in a completely different way.

    The RNA World theory relies on a simple idea: the most important thing a living organism can do is reproduce itself. Many biologists would agree with this. From bacteria to blue whales, all living things strive to have offspring.

    Wächtershäuser proposed that the first organisms were "drastically different from anything we know"

    However, many origin-of-life researchers do not believe reproduction is truly fundamental. Before an organism can reproduce, they say, it has to be self-sustaining. It must keep itself alive. After all, you cannot have kids if you die first.

    We keep ourselves alive by eating food, while green plants do it by extracting energy from sunlight. You might not think that a person wolfing down a juicy steak looks much like a leafy oak tree, but when you get right down to it, both are taking in energy.

    This process is called metabolism. First, you must obtain energy say, from energy-rich chemicals like sugars. Then you must use that energy to build useful things like cells.

    This process of harnessing energy is so utterly essential, many researchers believe it must have been the first thing life ever did.

    What might these metabolism-only organisms have looked like? One of the most influential suggestions was put forward in the late 1980s by Günter Wächtershäuser. He was not a full-time scientist, but rather a patent lawyer with a background in chemistry.

    Wächtershäuser proposed that the first organisms were "drastically different from anything we know". They were not made of cells. They did not have enzymes, DNA or RNA.

    All the other things that make up modern organisms &ndash like DNA, cells and brains &ndash came later

    Instead, Wächtershäuser imagined a flow of hot water streaming out of a volcano. The water was rich in volcanic gases like ammonia, and held traces of minerals from the volcano's heart.

    Where the water flowed over the rocks, chemical reactions began to take place. In particular, metals from the water helped simple organic compounds to fuse into larger ones.

    The turning point was the creation of the first metabolic cycle. This is a process in which one chemical is converted into a series of other chemicals, until eventually the original chemical is recreated. In the process, the entire system takes in energy, which can be used to restart the cycle &ndash and to start doing other things.

    Metabolic cycles may not seem life-like, but they are fundamental to life

    All the other things that make up modern organisms &ndash like DNA, cells and brains &ndash came later, built on the back of these chemical cycles.

    These metabolic cycles do not sound much like life. Wächtershäuser called his inventions "precursor organisms" and wrote that they "can barely be called living".

    But metabolic cycles like the ones Wächtershäuser described are at the core of every living thing. Your cells are essentially microscopic chemical processing plants, constantly turning one chemical into another. Metabolic cycles may not seem life-like, but they are fundamental to life.

    Over the 1980s and 1990s, Wächtershäuser worked out his theory in considerable detail. He outlined which minerals made for the best surfaces and which chemical cycles might take place. His ideas began to attract supporters.

    But it was all still theoretical. Wächtershäuser needed a real-world discovery that backed up his ideas. Fortunately, it had already been made &ndash a decade earlier.

    In 1977, a team led by Jack Corliss of Oregon State University took a submersible 1.5 miles (2.5km) down into the eastern Pacific Ocean. They were surveying the Galápagos hotspot, where tall ridges of rock rise from the sea floor. The ridges, they knew, were volcanically active.

    Each vent was a kind of primordial soup dispenser

    Corliss found that the ridges were pockmarked with, essentially, hot springs. Hot, chemical-rich water was welling up from below the sea floor and pumping out through holes in the rocks.

    Astonishingly, these "hydrothermal vents" were densely populated by strange animals. There were huge clams, limpets, mussels, and tubeworms. The water was also thick with bacteria. All these organisms lived on the energy from the hydrothermal vents.

    The discovery of hydrothermal vents made Corliss's name. It also got him thinking. In 1981 he proposed that similar vents existed on Earth four billion years ago, and that they were the site of the origin of life. He would spend much of the rest of his career working on this idea.

    Corliss proposed that hydrothermal vents could create cocktails of chemicals. Each vent, he said, was a kind of primordial soup dispenser.

    Key compounds like sugars "would survive&hellip for seconds at most"

    As hot water flowed up through the rocks, the heat and pressure caused simple organic compounds to fuse into more complex ones like amino acids, nucleotides and sugars. Closer to the boundary with the ocean, where the water was not quite as hot, they began linking into chains &ndash forming carbohydrates, proteins, and nucleotides like DNA. Then, as the water approached the ocean and cooled still further, these molecules assembled into simple cells.

    It was neat, and caught people's attention. But Stanley Miller, whose seminal origin-of-life experiment we discussed in Chapter One, was not convinced. Writing in 1988, he argued the vents were too hot.

    While extreme heat would trigger the formation of chemicals like amino acids, Miller's experiments suggested that it would also destroy them. Key compounds like sugars "would survive&hellip for seconds at most". What's more, these simple molecules would be unlikely to link up into chains, because the surrounding water would break the chains almost immediately.

    At this point the geologist Mike Russell stepped into the fray. He thought that the vent theory could be made to work after all. What's more, it seemed to him that the vents were the ideal home for Wächtershäuser's precursor organisms. This inspiration would lead him to create one of the most widely-accepted theories of the origin of life.

    If Russell was correct, life began at the bottom of the sea

    Russell had spent his early life variously making aspirin, scouting for valuable minerals and &ndash in one remarkable incident in the 1960s &ndash coordinating the response to a possible volcanic eruption, despite having no training. But his real interest was in how Earth's surface has changed over the eons. This geological perspective has shaped his ideas on the origin of life.

    In the 1980s he found fossil evidence of a less extreme kind of hydrothermal vent, where the temperatures were below 150C. These milder temperatures, he argued, would allow the molecules of life to survive far longer than Miller had assumed they would.

    What's more, the fossil remains of these cooler vents held something strange. A mineral called pyrite, which is made of iron and sulphur, had formed into tubes about 1mm across.

    In his lab, Russell found that the pyrite could also form spherical blobs. He suggested that the first complex organic molecules formed inside these simple pyrite structures.

    Around this time, Wächtershäuser had begun publishing his ideas, which relied on a stream of hot chemical-rich water flowing over a mineral. He had even proposed that pyrite was involved.

    His idea relied on the work of one of modern science's forgotten geniuses

    So Russell put two and two together. He suggested that hydrothermal vents in the deep sea, tepid enough for the pyrite structures to form, hosted Wächtershäuser's precursor organisms. If Russell was correct, life began at the bottom of the sea &ndash and metabolism appeared first.

    Russell set all this out in a paper published in 1993, 40 years after Miller's classic experiment. It did not get the same excited media coverage, but it was arguably more important. Russell had combined two seemingly separate ideas &ndash Wächtershäuser's metabolic cycles and Corliss's hydrothermal vents &ndash into something truly convincing.

    Just to make it even more impressive, Russell also offered an explanation for how the first organisms obtained their energy. In other words, he figured out how their metabolism could have worked. His idea relied on the work of one of modern science's forgotten geniuses.

    In the 1960s, the biochemist Peter Mitchell fell ill and was forced to resign from the University of Edinburgh. Instead, he set up a private lab in a remote manor house in Cornwall. Isolated from the scientific community, his work was partly funded by a herd of dairy cows. Many biochemists, including, initially, Leslie Orgel, whose work on RNA we discussed in Chapter Two, thought that his ideas were utterly ridiculous.

    We now know that the process Mitchell identified is used by every living thing on Earth

    Less than two decades later, Mitchell achieved the ultimate victory: the 1978 Nobel Prize in Chemistry. He has never been a household name, but his ideas are in every biology textbook.

    Mitchell spent his career figuring out what organisms do with the energy they get from food. In effect, he was asking how we all stay alive from moment to moment.

    He knew that all cells store their energy in the same molecule: adenosine triphosphate (ATP). The crucial bit is a chain of three phosphates, anchored to the adenosine. Adding the third phosphate takes a lot of energy, which is then locked up in the ATP.

    When a cell needs energy &ndash say, if a muscle needs to contract &ndash it breaks the third phosphate off an ATP. This turns it into adenosine diphosphate (ADP) and releases the stored energy.

    He has never been a household name

    Mitchell wanted to know how the cells made the ATP in the first place. How did they concentrate enough energy onto an ADP, so that the third phosphate would attach?

    Mitchell knew that the enzyme that makes ATP sits on a membrane. So he suggested that the cell was pumping charged particles called protons across the membrane, so that there were lots of protons on one side and hardly any on the other.

    The protons would then try to flow back across the membrane to balance out the number of protons on each side &ndash but the only place they could get through was the enzyme. The stream of protons passing through gave the enzyme the energy it needed to make ATP.

    See how cells harness energy in this video:

    Mitchell first set out this idea in 1961. He spent the next 15 years defending it from all comers, until the evidence became irrefutable. We now know that the process Mitchell identified is used by every living thing on Earth. It is happening inside your cells right now. Like DNA, it is fundamental to life as we know it.

    The key point that Russell picked up on is Mitchell's proton gradient: having lots of protons on one side of a membrane, and few on the other. All cells need a proton gradient to store energy.

    Modern cells create the gradients by pumping protons across a membrane, but this involves complex molecular machinery that cannot have just popped into existence. So Russell made one more logical leap: life must have formed somewhere with a natural proton gradient.

    Somewhere like a hydrothermal vent. But it would have to be a specific type of vent. When Earth was young the seas were acidic, and acidic water has a lot of protons floating around inside it. To create a proton gradient, the water from the vent must have been low in protons: it must have been alkaline.

    Corliss's vents would not do. Not only were they too hot, they were acidic. But in 2000, Deborah Kelley of the University of Washington discovered the first alkaline vents.

    Kelley had to battle just to become a scientist in the first place. Her father died as she was finishing high school, and she was forced to work long hours to support herself through college.

    He became convinced that vents like those of Lost City were where life began

    But she succeeded, and became fascinated both by undersea volcanoes and the searing hot hydrothermal vents. Those twin loves eventually led her to the middle of the Atlantic Ocean. There, Earth's crust is being pulled apart and a ridge of mountains rises from the sea floor.

    On this ridge, Kelley found a field of hydrothermal vents that she called "Lost City". They are not like the ones Corliss found. The water flowing from them is only 40-75C, and mildly alkaline. Carbonate minerals from this water have clumped into steep, white "chimneys" that rise from the sea bed like organ pipes. Their appearance is eerie and ghost-like, but this is misleading: they are home to dense communities of microorganisms that thrive on the vent water.

    These alkaline vents were the perfect fit for Russell's ideas. He became convinced that vents like those of Lost City were where life began.

    But he had a problem. Being a geologist, he did not know enough about biological cells to make his theory truly convincing.

    So Russell teamed up with biologist William Martin, a pugnacious American who has spent most of his career in Germany. In 2003 the pair set out an improved version of Russell's earlier ideas. It is arguably the most fleshed-out story of how life began.

    This story is now regarded as one of the leading hypotheses for the origin of life

    Thanks to Kelley, they now knew that the rocks of alkaline vents were porous: they were pocked with tiny holes filled with water. These little pockets, they suggested, acted as "cells". Each pocket contained essential chemicals, including minerals like pyrite. Combined with the natural proton gradient from the vent, they were the ideal place for metabolism to begin.

    Once life had harnessed the chemical energy of the vent water, Russell and Martin say, it started making molecules like RNA. Eventually it created its own membrane and became a true cell, and escaped from the porous rock into the open water.

    This story is now regarded as one of the leading hypotheses for the origin of life.

    It found powerful support in July 2016, when Martin published a study reconstructing some of the features of the "last universal common ancestor" (LUCA). This is the organism that lived billions of years ago and from which all existing life is descended.

    RNA World supporters say the vent theory has two problems

    We will probably never find direct fossil evidence of LUCA, but we can still make an educated guess as to how it might have looked and behaved by looking at microorganisms that do survive today. This is what Martin did.

    He examined the DNA of 1,930 modern microorganisms, and identified 355 genes that almost all of them had. This is arguably evidence that these 355 genes have been passed down, from generation to generation, ever since those 1,930 microbes shared a common ancestor &ndash roughly at the time that LUCA was alive.

    The 355 genes included some for harnessing a proton gradient, but not genes for generating one &ndash exactly as Russell and Martin's theories would predict. What's more, LUCA seems to have been adapted to the presence of chemicals like methane, which suggests it inhabited a volcanically-active environment &ndash like a vent.

    Despite this, RNA World supporters say the vent theory has two problems. One could potentially be fixed: the other might be fatal.

    The first problem is that there is no experimental evidence for the processes Russell and Martin describe. They have a step-by-step story, but none of the steps have been seen in a lab.

    "The people who think replication was first, they continuously provide new experimental data," says origin-of-life expert Armen Mulkidjanian. "The people who favour metabolism-first do not."

    The chemistry of all these molecules is incompatible with water

    That could change, thanks to Martin's colleague Nick Lane of University College London. He has built an "origin of life reactor", which will simulate the conditions inside an alkaline vent. He hopes to observe metabolic cycles, and perhaps even molecules like RNA. But it is early days.

    The second problem is the vents' location in the deep sea. As Miller pointed out in 1988, long-chain molecules like RNA and proteins cannot form in water without enzymes to help them.

    For many researchers, this is a knock-down argument. "If you have a background in chemistry, you cannot buy the idea of deep-sea vents, because you know the chemistry of all these molecules is incompatible with water," says Mulkidjanian.

    Regardless, Russell and his allies remain bullish.

    But in the last decade, a third approach has come to the fore, bolstered by a series of extraordinary experiments. This promises something that neither the RNA World nor the hydrothermal vents have so far managed: a way to make an entire cell from scratch.

    Chapter 5. How to make a cell

    By the early 2000s, there were two leading ideas about how life could have begun. Supporters of the "RNA World" were convinced that life began with a self-replicating molecule. Meanwhile, scientists in the "metabolism-first" camp had developed a detailed narrative about how life could have begun in hydrothermal vents in the deep sea. However, a third idea was about to come to the fore.

    Every living thing on Earth is made of cells. Each cell is basically a squishy ball, with a tough outer wall or "membrane".

    The point of a cell is to keep all the essentials of life together. If the outer wall gets torn open, the guts spill out and the cell dies &ndash just as a person who has been disembowelled generally does not have long to live.

    In the heat and tempest of the early Earth, a few raw materials must have assembled into crude cells

    The outer wall of the cell is so essential, some origin-of-life researchers argue that it must have been the first thing that emerged. They think that the "genetics first" efforts discussed in Chapter Three and the "metabolism first" ideas discussed in Chapter Four are misguided. Their alternative &ndash "compartmentalisation-first" &ndash has its champion in Pier Luigi Luisi of Roma Tre University in Rome, Italy.

    Luisi's reasoning is simple and hard to argue with. How could you possibly set up a working metabolism or a self-replicating RNA, each of which relies on having a lot of chemicals in one place, unless you first have a container to keep all the molecules in?

    If you accept this, there is only one way life could have begun. Somehow, in the heat and tempest of the early Earth, a few raw materials must have assembled into crude cells, or "protocells". The challenge is to make this happen in a lab: to create a simple living cell.

    Luisi can trace his ideas all the way back to Alexander Oparin and the dawn of origin-of-life science in the USSR &ndash discussed in Chapter One. Oparin highlighted the fact that certain chemicals form into blobs called coacervates, which can hold other substances in their cores. He suggested that these coacervates were the first protocells.

    The challenge was to make the protocells out of just the right stuff

    Any fatty or oily substance will form blobs or films in water. These chemicals are collectively known as lipids, and the idea that they formed the first life has been called the "Lipid World".

    But just forming blobs is not enough. The blobs need to be stable, they need to be able to divide to form "daughter" blobs, and they need at least some control over what travels in and out of them &ndash all without the elaborate proteins that modern cells use to achieve these things.

    The challenge was to make the protocells out of just the right stuff. Despite trying many substances over the decades, Luisi has never made anything lifelike enough to be convincing.

    Then in 1994, Luisi made a daring suggestion. He proposed that the first protocells must have contained RNA. What's more, this RNA must have been able to replicate inside the protocell.

    We would meet at origins meetings and get into these long arguments

    It was a big ask, and it meant abandoning the pure compartmentalisation-first approach. But Luisi had good reasons.

    A cell with an outer wall, but no genes inside it, could not do anything much. It might be able to divide into daughter cells, but it could not pass on any information about itself to its offspring. It could only start evolving and becoming more complex if it contained some genes.

    This idea would soon gain a crucial supporter in Jack Szostak, whose work on the RNA World hypothesis we explored in Chapter Three. While Luisi was a member of the compartmentalisation-first camp, Szostak supported genetics-first, so for many years they had not seen eye-to-eye.

    "We would meet at origins meetings and get into these long arguments about which was more important and which came first," recalls Szostak. "Eventually, we realised that cells have both. We came to a consensus that for the origin of life, it was critical to have both compartmentalisation and a genetic system."

    Szostak and two colleagues announced a major success

    In 2001, Szostak and Luisi set out their case for this more unified approach. Writing in Nature, they argued that it should be possible to make simple living cells from scratch, by hosting replicating RNAs in a simple, fatty blob.

    It was a dramatic idea, and Szostak soon decided to put his money where his mouth was. Reasoning that "we can't put out that theory without anything backing it up", he decided to start experimenting with protocells.

    Two years later, Szostak and two colleagues announced a major success.

    They had been experimenting with vesicles: spherical blobs, with two layers of fatty acids on the outside and a central core of liquid.

    Montmorillonite, and clays like it, could be important in the origin of life

    Trying to find a way to speed up the creation of the vesicles, they added small particles of a kind of clay called montmorillonite.

    This made the vesicles form 100 times faster. The surface of the clay acted as a catalyst, just like an enzyme would.

    What's more, the vesicles could absorb both montmorillonite particles and RNA strands from the clay surface. These protocells now contained genes and a catalyst, all from one simple setup.

    The decision to add montmorillonite was not done on a whim. Several decades of work had suggested that montmorillonite, and clays like it, could be important in the origin of life.

    Montmorillonite is a common clay. Nowadays it is used for all sorts of things, including making cat litter. It forms when volcanic ash is broken down by the weather. Since the early Earth had lots of volcanoes, it seems likely that montmorillonite was abundant.

    This had led Ferris to speculate that this ordinary-looking clay was the site of the origin of life. Szostak took that idea and ran with it, using montmorillonite to help build his protocells.

    If the protocells could grow, maybe they could also divide

    One year later, Szostak's team found that their protocells could grow of their own accord.

    As ever more RNA molecules were packed into a protocell, the outer wall came under increasing tension. It was as if the protocell had a full stomach and might go pop.

    To compensate, the protocell picked up more fatty acids and incorporated them into its wall, allowing it to swell to a larger size and releasing the tension.

    Crucially, it took the fatty acids from other protocells that contained less RNA, causing them to shrink. This meant the protocells were competing, and the ones with more RNA were winning.

    This suggested something even more impressive. If the protocells could grow, maybe they could also divide. Could Szostak's protocells reproduce themselves?

    Szostak's first experiments had shown a way to make protocells divide. Squeezing them through small holes stretched them out into tubes, which then broke into "daughter" protocells.

    The protocells grew and changed shape, elongating into long, rope-like strands

    This was neat, because no cellular machinery was involved: just the application of pressure. But it was not a great solution, because the protocells lost some of their contents in the process. It also implied that the first cells could only divide if they were pushed through tiny holes.

    There are lots of ways to make vesicles divide: for example, adding a strong water current that creates a shearing force. The trick was to make the protocells divide without spilling their guts.

    In 2009, Szostak and his student Ting Zhu found a solution. They made slightly more complex protocells, with several concentric outer walls a bit like the layers of an onion. Despite their intricacy, these protocells were still easy to make.

    As Zhu fed them with ever more fatty acids, the protocells grew and changed shape, elongating into long, rope-like strands. Once a protocell was long enough, a gentle shearing force was enough to make it shatter into dozens of small daughter protocells.

    Each daughter protocell contained RNAs from the parent protocell, and hardly any of the RNA was lost. What's more, the protocells could perform the cycle repeatedly, with daughter protocells growing and then dividing themselves.

    In later experiments, Zhu and Szostak have found even more ways to persuade the protocells to divide. This aspect of the problem, at least, seems to be solved.

    However, the protocells were still not doing enough. Luisi had wanted the protocells to host replicating RNA, but so far the RNA was simply sitting in them doing nothing.

    There were valuable clues buried in those dusty papers

    To really show that his protocells could have been the first life on Earth, Szostak needed to persuade the RNA inside them to replicate itself.

    That was not going to be easy, because despite decades of trying &ndash outlined in Chapter Three &ndash nobody had managed to make an RNA that could self-replicate. That was the very problem that had stymied Szostak in his early work on the RNA World, and which nobody else had managed to solve.

    So he went back and re-read the work of Leslie Orgel, who had spent so long working on the RNA World hypothesis. There were valuable clues buried in those dusty papers.

    Orgel had spent much the 1970s and 1980s studying how RNA strands get copied.

    This could have been how the first life made copies of its genes

    In essence it is simple. Take a single strand of RNA and a pool of loose nucleotides. Then, use those nucleotides to assemble a second strand of RNA that is complementary to the first one.

    For example, a strand of RNA that reads "CGC" will produce a complementary strand that reads "GCG". If you do this twice, you will get a copy of the original "CGC", just in a roundabout way.

    Orgel found that, under certain circumstances, RNA strands could copy in this way without any help from enzymes. This could have been how the first life made copies of its genes.

    By 1987, Orgel could take an RNA strand 14 nucleotides long and create complementary strands that were also 14 nucleotides long. He did not manage anything longer, but that was enough to intrigue Szostak. His student Katarzyna Adamala tried to get this reaction going in the protocells.

    They have built protocells that hold onto their genes while taking in useful molecules from outside

    They found that the reaction needed magnesium to work, which was a problem because the magnesium destroyed the protocells. But there was a simple solution: citrate, which is almost identical to the citric acid in lemons and oranges, and which is found in all living cells anyway.

    In a study published in 2013, they added citrate and found that it latched onto the magnesium, protecting the protocells while allowing the template copying to continue.

    In other words, they had achieved what Luisi had proposed in 1994. "We started to do RNA replication chemistry inside these fatty acid vesicles," says Szostak.

    In just over a decade of research, Szostak's team has accomplished something remarkable.

    They have built protocells that hold onto their genes while taking in useful molecules from outside. The protocells can grow and divide, and even compete with each other. RNA can replicate inside them. By any measure, they are startlingly life-like.

    Szostak's approach went against 40 years of work on the origin of life

    They are also resilient. In 2008, Szostak's team found that the protocells could survive being heated to 100C, a temperature that would obliterate most modern cells. This boosted the case that the protocells were similar to the first life, which must have endured scalding heat from constant meteor impacts.

    "Szostak is doing great work," says Armen Mulkidjanian.

    Yet on the face of it, Szostak's approach went against 40 years of work on the origin of life. Instead of focusing on "replication-first" or "compartmentalisation-first", he found ways to get both to happen pretty much simultaneously.

    That would inspire a new unified approach to the origin of life, which attempts to jumpstart all the functions of life at once. This "everything-first" idea has already accumulated a wealth of evidence, and could potentially solve all the problems with the existing ideas.

    Chapter 6. The great unification

    Throughout the second half of the 20th Century, origin-of-life researchers have worked in tribes. Each group favoured their own narrative and, for the most part, rubbished competing hypotheses. This approach has certainly been successful, as evidenced by the previous chapters, but every promising idea for the origin of life has ultimately come up against a major problem. So a few researchers are now trying a more unified approach.

    This idea got its first big boost a few years ago from a result that, on the face of it, seemed to support the traditional, replication-first RNA World.

    All the key components of life could be formed at once

    By 2009, supporters of the RNA World had a big problem. They could not make nucleotides, the building blocks of RNA, in a way that could plausibly have happened on the early Earth. This, as we learned in Chapter Three, led people to suspect that the first life was not based on RNA at all.

    John Sutherland had been thinking about this problem since the 1980s. "I thought, if you could demonstrate that RNA could self-assemble that would be a cool thing to do," he says.

    Fortunately for Sutherland, he had secured a job at the Laboratory of Molecular Biology (LMB) in Cambridge, UK. Most research institutions force their staff to constantly churn out new findings, but the LMB does not. So Sutherland could think about why it was so hard to make an RNA nucleotide, and to spend years developing an alternative approach.

    His solution would lead him to propose a radical new idea about the origin of life, namely that all the key components of life could be formed at once.

    "There were certain key aspects of RNA chemistry that didn't work," says Sutherland. Each RNA nucleotide is made of a sugar, a base and a phosphate. But it had proved impossible to persuade the sugar and base to join up. The molecules were simply the wrong shape.

    He believes RNA was heavily involved, but it was not the be-all-and-end-all

    So Sutherland started trying totally different substances. Eventually his team homed in on five simple molecules, including a different sugar and cyanamide, which as the name suggests is related to cyanide. The team put these chemicals through a series of reactions that ultimately produced two of the four RNA nucleotides, without ever making standalone sugars or bases.

    It was a slam-dunk success, and it made Sutherland's name.

    Many observers interpreted the findings as further evidence for the RNA World. But Sutherland himself does not see it like that at all.

    The "classic" RNA World hypothesis says that, in the first organisms, RNA was responsible for all the functions of life. But Sutherland says that is "hopelessly optimistic". He believes RNA was heavily involved, but it was not the be-all-and-end-all.

    The molecules were simply the wrong shape

    Instead, he takes inspiration from the recent work of Jack Szostak, which &ndash as discussed in Chapter Five &ndash combines the "replication-first" RNA World with Pier Luigi Luisi's "compartmentalisation-first" ideas.

    But Sutherland goes further. His approach is "everything-first". He aims to make an entire cell assemble itself, from scratch.

    His first clue was an odd detail about his nucleotide synthesis, which at first seemed incidental.

    The last step in Sutherland's process was to bolt a phosphate onto the nucleotide. But he found that it was best to include the phosphate in the mix right from the start, because it accelerated the earlier reactions.

    On the face of it, including the phosphate before it was strictly needed was a messy thing to do, but Sutherland found that this messiness was a good thing.

    Get the mixture just complicated enough and all the components of life might form at once

    This led him to think about how messy his mixtures should be. On the early Earth, there must have been dozens or hundreds of chemicals all floating around together. That sounds like a recipe for a sludge, but maybe there was an optimum level of mess.

    The mixtures Stanley Miller made back in the 1950s, which we looked at in Chapter One, were far messier than Sutherland's. They did contain biological molecules, but Sutherland says they "were in trace amounts and they were accompanied by a vast number of other compounds, which are not biological".

    For Sutherland, this meant that Miller's setup was not good enough. It was too messy, so the good chemicals got lost in the mixture.

    So Sutherland has set out to find a "Goldilocks chemistry": one that is not so messy that it becomes useless, but also not so simple that it is limited in what it can do. Get the mixture just complicated enough and all the components of life might form at once, then come together.

    In other words, four billion years ago there was a pond on the Earth. It sat there for years until the mix of chemicals was just right. Then, perhaps within minutes, the first cell came into existence.

    This may sound implausible, like the claims of medieval alchemists. But Sutherland's evidence is mounting. Since 2009, he has shown that the same chemistry that made his two RNA nucleotides can also make many of the other molecules of life.

    Our entire approach to the origin of life for the last 40 years has been wrong

    The obvious next step was to make more RNA nucleotides. He has not yet managed this, but in 2010 he made closely-related molecules that could potentially transform into the nucleotides.

    Similarly, in 2013 he made the precursors of amino acids. This time he needed to add copper cyanide to make the reactions go.

    Cyanide-related chemicals were proving to be a common theme, and in 2015 Sutherland took them even further. He showed that the same pot of chemicals could also produce the precursors of lipids, the molecules that make up cell walls. The reactions were all driven by ultraviolet light, involved sulphur, and relied on copper to speed them up.

    "All the building blocks [emerge] from a common core of chemical reactions," says Szostak.

    The experiments were too clean

    If Sutherland is right, then our entire approach to the origin of life for the last 40 years has been wrong. Ever since the sheer complexity of the cell became clear, scientists have been working on the assumption that the first cells must have been constructed gradually, one piece at a time.

    Following Leslie Orgel's proposal that RNA came first, researchers have been "trying to get one thing before another thing, and then have that invent the other", says Sutherland. But he thinks the best way is to make everything at once.

    "What we've done is to challenge the idea that it's too complicated to make everything in one go," says Sutherland. "You certainly could make the building blocks for all the systems at once."

    Szostak now suspects that most attempts to make the molecules of life, and to assemble them into living cells, have failed for the same reason: the experiments were too clean.

    I've really come back to the idea that the first polymer was something pretty close to RNA

    The scientists used the handful of chemicals they were interested in, and left out all the other ones that were probably present on the early Earth. But Sutherland's work shows that, by adding a few more chemicals to the mix, more complex phenomena can be created.

    Szostak experienced this for himself in 2005, when he was trying to get his protocells to host an RNA enzyme. The enzyme needed magnesium, which destroyed the protocells' membranes.

    The solution was a surprising one. Instead of making the vesicles out of one pure fatty acid, they made them from a mixture of two. These new, impure vesicles could cope with the magnesium &ndash and that meant they could play host to working RNA enzymes.

    What's more, Szostak says the first genes might also have embraced messiness.

    Modern organisms use pure DNA to carry their genes, but pure DNA probably did not exist at first. There would have been a mixture of RNA nucleotides and DNA nucleotides.

    In 2012 Szostak showed that such a mixture could assemble into "mosaic" molecules that looked and behaved pretty much like pure RNA. These jumbled RNA/DNA chains could even fold up neatly.

    There is one problem that neither Sutherland nor Szostak have found a solution for

    This suggested that it did not matter if the first organisms could not make pure RNA, or pure DNA. "I've really come back to the idea that the first polymer was something pretty close to RNA, a messier version of RNA," says Szostak.

    There might even be room for the alternatives to RNA that have been cooked up in labs, like the TNA and PNA we met in Chapter Three. We do not know if any of them ever existed on Earth, but if they did the first organisms may well have used them alongside RNA.

    This was not an RNA World: it was a "Hodge-Podge World".

    The lesson from these studies is that making the first cell might not have been as hard as it once seemed. Yes, cells are intricate machines. But it turns out that they still work, albeit not quite as well, when they are flung together slapdash from whatever is to hand.

    Such clumsy cells might seem unlikely to survive on the early Earth. But they would not have had much competition, and there were no threatening predators, so in many respects life may have been easier then than it is now.

    There is one problem that neither Sutherland nor Szostak have found a solution for, and it is a big one. The first organism must have had some form of metabolism. Right from the start, life had to obtain energy or it would have died.

    Life may have been easier then than it is now

    On that point, if on nothing else, Sutherland agrees with Mike Russell, Bill Martin and the other supporters of Chapter Four's metabolism-first theories. "While the RNA guys were fighting with the metabolism guys, both sides had a point," says Sutherland.

    "The origins of metabolism have to be in there somehow," says Szostak. "The source of chemical energy is going to be the big question."

    Even if Martin and Russell are wrong about life beginning in deep-sea vents, many elements of their theory are almost certainly correct. One is the importance of metals for the birth of life.

    In nature, many enzymes have a metal atom at their core. This is often the "active" part of the enzyme, with the rest of the molecule essentially a support structure. The first life cannot have had these complex enzymes, so instead it probably used "naked" metals as catalysts.

    Life cannot have begun in the deep sea

    Günter Wächtershäuser made this point when he suggested that life formed on iron pyrite. Similarly, Russell emphasises that the waters of hydrothermal vents are rich in metals, which could act as catalysts &ndash and Martin's study of LUCA found a lot of iron-based enzymes.

    In light of this, it is telling that many of Sutherland's chemical reactions rely on copper (and, incidentally, on the sulphur that Wächtershäuser also emphasised), and that the RNA in Szostak's protocells needs magnesium.

    It may yet be that hydrothermal vents will turn out to be crucial. "If you look at modern metabolism, there's all these really suggestive things like iron-sulphur clusters," says Szostak. That fits the idea that life began in or around a vent, where the water is rich in iron and sulphur.

    That said, if Sutherland and Szostak are on the right track, one aspect of the vent theory is definitely wrong: life cannot have begun in the deep sea.

    "The chemistry we've uncovered is so dependent on UV [ultraviolet light]," says Sutherland. The only source of ultraviolet radiation is the Sun, so his reactions can only take place in sunny places. "It rules out a deep-sea vent scenario."

    Maybe life began on land, in a volcanic pond

    Szostak agrees that the deep sea was not life's nursery. "The worst thing is that it's isolated from atmospheric chemistry, which is the source of high-energy starting materials like cyanide."

    But these problems do not rule out hydrothermal vents altogether. Perhaps the vents were simply in shallow water, where sunlight and cyanide could reach them.

    Armen Mulkidjanian has suggested an alternative. Maybe life began on land, in a volcanic pond.

    Mulkidjanian looked at the chemical makeup of cells: specifically, which chemicals they allow in and which they keep out. It turns out that all cells, regardless of what organism they belong to, contain a lot of phosphate, potassium and other metals &ndash but hardly any sodium.

    My favourite scenario at the moment would be some kind of shallow lakes or ponds on the surface

    Nowadays, cells achieve this by pumping things in and out, but the first cells cannot have done so because they would not have had the necessary machinery. So Mulkidjanian suggested that the first cells formed somewhere that had roughly the same mix of chemicals as modern cells.

    That immediately eliminates the ocean. Cells contain far higher levels of potassium and phosphate than the ocean ever has, and far less sodium.

    Instead, it points to the geothermal ponds found near active volcanoes. These ponds have exactly the cocktail of metals found in cells.

    Szostak is a fan. "I think my favourite scenario at the moment would be some kind of shallow lakes or ponds on the surface, in a geothermally-active area," he says. "You have hydrothermal vents but not like the deep-sea vents, more like the kind of vents we have in volcanic areas like Yellowstone."

    Earth was pounded by meteorites throughout its first half-billion years of existence

    Sutherland's chemistry might well work in such a place. The springs have the right chemicals, the water level fluctuates so some places will dry out at times, and there is plenty of ultraviolet radiation from the Sun.

    What's more, Szostak says the ponds would be suitable for his protocells.

    "The protocells could be relatively cool most of the time, which is good for RNA copying and other kinds of simple metabolism," says Szostak. "But every now and then they get heated up briefly, and that helps the strands of RNA come apart ready for the next round of replication." There would also be currents, driven by the streams of hot water, which could help the protocells divide.

    Drawing on many of the same lines of argument, Sutherland has put forward a third option: a meteorite impact zone.

    Earth was pounded by meteorites throughout its first half-billion years of existence &ndash and has been occasionally struck ever since. A decent-sized impact would create a setup rather similar to Mulkidjanian's ponds.

    First, meteorites are mostly made of metal. The impact zones tend to be rich in useful metals like iron, as well as sulphur. And crucially, meteorite impacts melt the Earth's crust, leading to geothermal activity and hot water.

    If it turns out that one of the scenarios is missing a key chemical, or contains something that destroys protocells, it will be ruled out

    Sutherland imagines small rivers and streams trickling down the slopes of an impact crater, leaching cyanide-based chemicals from the rocks while ultraviolet radiation pours down from above. Each stream would have a slightly different mix of chemicals, so different reactions would happen and a whole host of organic chemicals would be produced.

    Finally the streams would flow into a volcanic pond at the bottom of the crater. It could have been in a pond like this that all the pieces came together and the first protocells formed.

    "That's a very specific scenario," says Sutherland. But he chose it on the basis of the chemical reactions he has found. "It's the only one we can think of that's compatible with the chemistry."

    Szostak is not sure either way, but he agrees that Sutherland's idea deserves careful attention. "I think the impact scenario is nice. I think the idea of volcanic systems might also work. There's some arguments in favour of each."

    For now that debate looks set to rumble on. But it will not be decided on a whim. The decision will be driven by the chemistry and the protocells. If it turns out that one of the scenarios is missing a key chemical, or contains something that destroys protocells, it will be ruled out.

    This means that, for the first time in history, we have the beginnings of a comprehensive explanation for how life began.

    "Things are looking a lot more achievable," says Sutherland.

    The best we can ever do is to draw up a story that is consistent with all the evidence

    So far, the "everything-at-once" approach of Szostak and Sutherland offers only a sketchy narrative. But those steps that have been worked out are supported by decades of experiments.

    The idea also draws on every approach to the origin of life. It attempts to harness all their good points, while at the same time solving all their problems. For instance, it does not so much try to disprove Russell's ideas about hydrothermal vents, but rather to incorporate their best elements.

    We cannot know for sure what happened four billion years ago. "Even if you made a reactor and out pops E. coli on the other side&hellip you still can't prove that we arose that way," says Martin.

    The best we can ever do is to draw up a story that is consistent with all the evidence: with experiments in chemistry, with what we know about the early Earth, and with what biology reveals about the oldest forms of life. Finally, after a century of fractious effort, that story is coming into view.

    That means we are approaching one of the great divides in human history: the divide between those who know the story of life's beginning, and those who never could.

    Some of the people alive today will become the first in history who can honestly say they know where they came from

    Every single person who died before Darwin published Origin of Species in 1859 was ignorant of humanity's origins, because they knew nothing of evolution. But everyone alive now, barring isolated groups, can know the truth about our kinship with other animals.

    Similarly, everyone born after Yuri Gagarin orbited the Earth in 1961 has lived in a society that can travel to other worlds. Even if we never go ourselves, space travel is a reality.

    These facts change our worldview in subtle ways. Arguably, they make us wiser. Evolution teaches us to treasure every other living thing, for they are our cousins. Space travel allows us to see our world from a distance, revealing how unique and fragile it is.

    Some of the people alive today will become the first in history who can honestly say they know where they came from. They will know what their ultimate ancestor was like and where it lived.

    This knowledge will change us. On a purely scientific level, it will tell us about how likely life is to form in the Universe, and where to look for it. And it will tell us something about life's essential nature. But beyond that, we cannot yet know the wisdom the origin of life will reveal.

    Join over five million BBC Earth fans by liking us on Facebook, or follow us on Twitter and Instagram.


    Meteorite Yields Evidence of Primitive Life on Early Mars

    The NASA-funded team found the first organic molecules thought to be of Martian origin several mineral features characteristic of biological activity and possible microscopic fossils of primitive, bacteria-like organisms inside of an ancient Martian rock that fell to Earth as a meteorite. This array of indirect evidence of past life will be reported in the August 16 issue of the journal Science, presenting the investigation to the scientific community at large for further study.

    The two-year investigation was co-led by JSC planetary scientists Dr. David McKay, Dr. Everett Gibson and Kathie Thomas-Keprta of Lockheed-Martin, with the major collaboration of a Stanford team headed by Professor of Chemistry Dr. Richard Zare, as well as six other NASA and university research partners.

    "There is not any one finding that leads us to believe that this is evidence of past life on Mars. Rather, it is a combination of many things that we have found," McKay said. "They include Stanford's detection of an apparently unique pattern of organic molecules, carbon compounds that are the basis of life. We also found several unusual mineral phases that are known products of primitive microscopic organisms on Earth. Structures that could be microsopic fossils seem to support all of this. The relationship of all of these things in terms of location - within a few hundred thousandths of an inch of one another - is the most compelling evidence."

    "It is very difficult to prove life existed 3.6 billion years ago on Earth, let alone on Mars," Zare said. "The existing standard of proof, which we think we have met, includes having an accurately dated sample that contains native microfossils, mineralogical features characteristic of life, and evidence of complex organic chemistry."

    "For two years, we have applied state-of-the-art technology to perform these analyses, and we believe we have found quite reasonable evidence of past life on Mars," Gibson added. "We don't claim that we have conclusively proven it. We are putting this evidence out to the scientific community for other investigators to verify, enhance, attack -- disprove if they can -- as part of the scientific process. Then, within a year or two, we hope to resolve the question one way or the other."

    "What we have found to be the most reasonable interpretation is of such radical nature that it will only be accepted or rejected after other groups either confirm our findings or overturn them," McKay added.

    The igneous rock in the 4.2-pound, potato-sized meteorite has been age-dated to about 4.5 billion years, the period when the planet Mars formed. The rock is believed to have originated underneath the Martian surface and to have been extensively fractured by impacts as meteorites bombarded the planets in the early inner solar system. Between 3.6 billion and 4 billion years ago, a time when it is generally thought that the planet was warmer and wetter, water is believed to have penetrated fractures in the subsurface rock, possibly forming an underground water system.

    Since the water was saturated with carbon dioxide from the Martian atmosphere, carbonate minerals were deposited in the fractures. The team's findings indicate living organisms also may have assisted in the formation of the carbonate, and some remains of the microscopic organisms may have become fossilized, in a fashion similar to the formation of fossils in limestone on Earth. Then, 16 million years ago, a huge comet or asteroid struck Mars, ejecting a piece of the rock from its subsurface location with enough force to escape the planet. For millions of years, the chunk of rock floated through space. It encountered Earth's atmosphere 13,000 years ago and fell in Antarctica as a meteorite.

    It is in the tiny globs of carbonate that the researchers found a number of features that can be interpreted as suggesting past life. Stanford researchers found easily detectable amounts of organic molecules called polycyclic aromatic hydrocarbons (PAHs) concentrated in the vicinity of the carbonate. Researchers at JSC found mineral compounds commonly associated with microscopic organisms and the possible microscopic fossil structures.

    The largest of the possible fossils are less than 1/100 the diameter of a human hair, and most are about 1/1000 the diameter of a human hair - small enough that it would take about a thousand laid end-to-end to span the dot at the end of this sentence. Some are egg-shaped while others are tubular. In appearance and size, the structures are strikingly similar to microscopic fossils of the tiniest bacteria found on Earth.

    The meteorite, called ALH84001, was found in 1984 in Allan Hills ice field, Antarctica, by an annual expedition of the National Science Foundation's Antarctic Meteorite Program. It was preserved for study in JSC's Meteorite Processing Laboratory and its possible Martian origin was not recognized until 1993. It is one of only 12 meteorites identified so far that match the unique Martian chemistry measured by the Viking spacecraft that landed on Mars in 1976. ALH84001 is by far the oldest of the 12 Martian meteorites, more than three times as old as any other.

    Many of the team's findings were made possible only because of very recent technological advances in high- resolution scanning electron microscopy and laser mass spectrometry. Only a few years ago, many of the features that they report were undetectable. Although past studies of this meteorite and others of Martian origin failed to detect evidence of past life, they were generally performed using lower levels of magnification, without the benefit of the technology used in this research. The recent discovery of extremely small bacteria on Earth, called nanobacteria, prompted the team to perform this work at a much finer scale than past efforts.

    The nine authors of the Science report include McKay, Gibson and Thomas-Keprta of JSC Christopher Romanek, formerly a National Research Council post-doctoral fellow at JSC who is now a staff scientist at the Savannah River Ecology Laboratory at the University of Georgia Hojatollah Vali, a National Research Council post-doctoral fellow at JSC and a staff scientist at McGill University, Montreal, Quebec, Canada and Zare, graduate students Simon J. Clemett and Claude R. Maechling and post-doctoral student Xavier Chillier of the Stanford University Department of Chemistry.

    The team of researchers includes a wide variety of expertise, including microbiology, mineralogy, analytical techniques, geochemistry and organic chemistry, and the analysis crossed all of these disciplines. Further details on the findings presented in the Science article include:

      Researchers at Stanford University used a dual laser mass spectrometer -- the most sensitive instrument of its type in the world -- to look for the presence of the common family of organic molecules called PAHs. When microorganisms die, the complex organic molecules that they contain frequently degrade into PAHs. PAHs are often associated with ancient sedimentary rocks, coals and petroleum on Earth and can be common air pollutants. Not only did the scientists find PAHs in easily detectable amounts in ALH84001, but they found that these molecules were concentrated in the vicinity of the carbonate globules. This finding appears consistent with the proposition that they are a result of the fossilization process. In addition, the unique composition of the meteorite's PAHs is consistent with what the scientists expect from the fossilization of very primitive microorganisms. On Earth, PAHs virtually always occur in thousands of forms, but, in the meteorite, they are dominated by only about a half-dozen different compounds. The simplicity of this mixture, combined with the lack of light- weight PAHs like napthalene, also differs substantially from that of PAHs previously measured in non-Martian meteorites.