What is life? Even though an answer has been passed on to generations of biology students, they weren’t getting the full story. When scientists invented the modern field of astrobiology, they had to wrestle with a fundamental problem: There is no scientific consensus about precisely what makes something alive. Given that unsettling absence, did it really make sense for astrobiologists to apply to the rest of the universe the never-quite-exact definitions we had come up with on Earth? To make matters all the more confounding, what would be a sure signature of biology on our planet could be totally nonbiological on Mars, and vice versa. So how do you find life in the beyond if you can’t agree on what life is on Earth?
Because it’s in the business of trying to find “life” beyond Earth, NASA has probably done more to try to define it than any other organization. Here is an unofficial working definition: Life is “a self-sustaining chemical system with the capacity to evolve in a Darwinian manner.” The definition came out of a workshop of biologists, physicists, and chemists in 1994, and it does meet many of the basic criteria scientists and others are looking for. Broadly, it accounts for the known constants of life on Earth. All living organisms take in some form of energy, use and change it, and then release it as waste; all use the same twenty amino acids to construct the proteins that make that and all other activity possible; and all use RNA and DNA molecules to store genetic information and to construct proteins. The Darwinian evolution comes directly and inevitably from the presence of DNA, since all DNA mutates.
But the definition has many critics, some of whom think it is not only incorrect but also misguided. The criticisms come from many directions: those who argue the definition would rule out viruses, prions (which cause “mad cow” disease), and other seemingly “living” organisms; those who want to base any definition on a specific capability such as metabolism or reproduction or the enclosing of a cell nucleus by a cell wall; those who think in the more abstract terms of a physicist and want a definition that takes their discipline (the Second Law of Thermodynamics, for one) into account. Relying on that law, the Austrian quantum physicist Erwin Schrödinger famously suggested that life—in its broadest terms—be defined as something that avoids immediate decay into “entropy,” the chaotic and then utterly uniform state the entire universe will someday revert to since all structure has in it the seeds of its own falling apart. Living things, Schrödinger proposed in his 1944 book, What is Life?, postpone this inevitable process by taking in nutrients and turning them into energy; at death the life forms eventually succumb to the force of entropy and break down so the atoms of the once-living body become evenly distributed again, recycled by the Earth.
Portland State University geobiologist Radu Popa, author of the 2004 book Between Necessity and Probability: Searching for the Definition and Origin of Life, said that he lost count of the proposed answers in the scientific literature after logging at least three hundred. And the definitions keep coming. Nilton Renno, a planetary and atmospheric scientist at the University of Michigan and a member of the Mars Phoenix lander science team, recently came up with this one in a paper on the likelihood that the heat from the spacecraft’s landing created liquid water that remained visible for days: Life, he wrote, is a self-replicating heat engine with a capacity for mutation.
Perhaps the most subversive challenge to the proposed definitions of life comes not from those who think the NASA definition is incorrect, but rather from those who think “life” is not a concept we can or should define. Philosophy professor Carol Cleland, from the University of Colorado, and Chris Chyba, an astronomy student of Carl Sagan’s who now teaches at Princeton University, have argued for almost a decade that current definitions of “life” are little different from medieval definitions of “water,” which was seen then as a clear liquid with certain qualities such as wetness, transparency, tastelessness, odorlessness, and the property of being a very good solvent. We can now chuckle at the misunderstanding, since muddy water is certainly not transparent, salty water has a taste, and marshy water has a smell. Medieval alchemists classified nitric acid and some mixtures of hydrochloric acid as aqua fortis (strong water) and aqua regia (royal water) because they were such good solvents.
But water, as we now know, is H2O—two hydrogen atoms bound to one oxygen atom. Those men and women trying over the centuries to define water knew nothing about the molecules and atoms that we now know make up all matter. That didn’t come until the late eighteenth century, when Antoine Lavoisier came up with the convincing theory that matter is made up of molecules. Cleland, Chyba, and others have argued that the basic knowledge needed to make a definition of “life” is simply absent, rather like how the essential molecular nature of water was unknown during the Middle Ages. Based on her iconoclastic views—grounded in philosophy and at times a challenge to scientists—Cleland was included on a University of Colorado astrobiology team that was twice funded by NASA. Her thinking became more broadly known when she addressed a 2001 meeting called “The Nature of Life,” hosted by the American Association for the Advancement of Science. She told the audience of scientists that the search for a definition of life—something many were involved in—was a waste of time and, even worse, misleading.
“The logic of my argument was impeccable, but people just blew up at me,” recalls Cleland, an expert in the philosophy of science. It was a memorable evening. “They were yelling out their own definitions, saying this is the right definition or that is the right definition. It’s as if they totally missed my point that their approach was mistaken and there is no definition available now. I was kind of shocked and remember saying to myself, ‘These people just can’t hear what I’m saying.’ I’ve learned since then how to better talk with scientists, but I still think the whole definition project is hopeless.”
Ten years later, Cleland and Chyba’s view is no longer outlandish. Addressing a NASA-NSF gathering of many of the nation’s top practitioners of “synthetic biology” (the origins-of-life side of biotechnology), the evolutionary biologist Andrew Ellington, of the University of Texas at Austin, urged NASA to bring together a blue-ribbon panel to study and then throw out the agency’s and all other definitions of life.
“It is my position that there is no such thing as life, and that the working statement in the NASA document does science a disservice by attempting to pretend the contrary,” he told the gathering of in 2008. “‘Life’ is a term better suited for poets (or perhaps philosophers) than scientists, and the continuing attempts to determine whether a given system is alive or not harken back to quite ancient philosophers, with a similar level of resolution. I assert the following existence proof: if we haven’t figured out what life is by now, there is little hope that we will figure out a definitive definition in the near term, and there is no research program that I can imagine, at any price, that will provide such a definition.”
Ellington then made clear why he felt as strongly as he did. As is so often the case in astrobiology, the purely scientific issues are surrounded by deeply felt and highly contentious social and even political issues. “I would further argue that the reason that what is nominally a rather pointless philosophical issue has become an important one for NASA is because of its near-term political ramifications,” he said. He believes defining “life” is a dangerous endeavor because the information collected will almost inevitably weigh down science. “I can imagine a day when the head of NASA would be brought before the Supreme Court in an abortion case and asked to define life,” he told me. “And I can imagine the long and uncomfortable silence that would follow.” Let the work progress on synthesizing molecules that can do what living molecules do, and on determining if some unexpected substances have lifelike qualities, he says. But leave the definitions for later.
The controversy over a definition for “life” has actually been around for some time, even inside NASA, and it became a serious problem and even embarrassment in 1976 when the agency landed two Viking spacecrafts on Mars in a self-described search for life. To the initial delight of the Viking scientists, a key biology experiment on both Viking landers gave a strong signal that “life” had been found—meeting the painstakingly crafted criteria established before the spacecraft left Earth—and the control experiments seemed to confirm the finding. Yet the principal investigator of that experiment was held back from announcing what Viking had apparently discovered. The scientific community and NASA quickly formed a consensus that life had not been detected. The problem wasn’t with the way the instruments performed or how the experiment was carried out, but rather with the definition of life that NASA itself had put together, one based on the way metabolism is known to work on Earth.
The story is best told through the life and times of Gilbert V. Levin, a pioneer of astrobiology who began his career as a sanitary engineer searching for microbes in drinking water. He first proposed a life-detection experiment for Mars in 1959 and had his idea embraced and tested time and again by NASA before the Viking launch in 1975. He got the results he had dreamed of within ten days of the first landing of Viking. It seemed like a scientific triumph of historic proportions, but it quickly slipped away and Levin has been fighting ever since to reclaim the victory. More than ever, he says, he is convinced that his Viking experiment did find something that indeed was—had to be—living. But the scientific verdict came down against him and, despite some converts, has not significantly changed.
Levin’s experiment was conceptually quite simple: It added a number of liquid nutrients that had been “labeled” with radioactive carbon 14 to a sample of Martian soil dug up by the Viking collecting arm and pulled into the spacecraft. If these nutrients were eaten by Martian bacteria or other life forms, the gases they would inevitably release as waste would also be radioactively labeled and would be detected by an installed radiation counter. It was a simple and powerful test for a cornerstone of all definitions of “life”—the ability of an organism to use the chemicals contained in food to produce the energy it needs to maintain itself, to grow, and to reproduce. If radioactive gases were released, Levin and his NASA collaborators initially agreed, then an organism had taken in and broken down the nutrient food, and was passing the waste out when it was done. Thus the experiment’s name: Labeled Release.
After the nutrient was squirted into the soil collected on Mars, the monitoring instruments registered a surging amount of radioactive carbon dioxide gas—strongly suggesting that some organism had eaten the food and then released the gas. A follow-up control experiment heated the soil to a high temperature that would presumably kill any living organisms, and then squirted in the nutrient. This time there was no release of CO2, an apparent confirmation that the gas had been produced by the actions of an organism that had been alive during the first experiment but was killed by the heat in the second. Viking 2 landed four thousand miles away on Mars a month-and-a-half later, and the same Labeled Release experiment was conducted. Again, the radioactive gas was detected when food was delivered to the Martian soil at what amounted to room temperature, but not after samples of the same batch of soil were heated and cooked, or when it had been stored in a dark container for several months. It certainly seemed that metabolism—a process only known to occur in living organisms—was taking place. Two other Viking biology experiments got strong reactions when food was presented in gaseous form to the soil, but the controlled versions failed to support the results. Scientists quickly concluded the reactions came from chemical, and not biological, sources. Nonetheless, Levin was convinced that he had found life on Mars.
NASA was skittish about Levin’s results from the start. Officials cautioned that all the reactions could be chemical rather than biological, and that the speed of the appearance of radioactive CO2 did not appear consistent with a biological reaction (although they admitted it wasn’t consistent with a known chemical reaction, either). What they needed to make a firm scientific judgment was the data coming from another key experiment, one designed to determine whether organic compounds—the carbon-, hydrogen-, and oxygen-based molecules essential to all life on earth—were present in the soil. That experiment used a gas chromatograph mass spectrometer (GCMS) to heat the soil until chemicals turned to vapor, and then it separated, identified, and quantified the large number of different chemicals found. The device, refined and operated by prominent MIT biochemist Klaus Biemann, was designed to measure molecules present at a level of only a few parts per billion. The Viking arm twice failed to bring in soil for the GCMS, and so NASA and the many Viking watchers had to wait for days before the testing could begin.
When the samples did arrive, the results were both surprising and seemingly unequivocal: The instrument measured no indigenous organic molecules in the soil, indicating that Martian soil had even less carbon in it than the barren lunar soils brought to Earth during the Apollo program. The strongest organic concentrations it measured were minute trace chlorine-based organics written off as contaminants brought from Earth. Without organics, the scientists concluded, there could not be life, and so any experiment suggesting otherwise had to be reinterpreted. The anticipation that Viking just might delight the world by finding life on Mars quickly turned to a conviction that Mars was lifeless—without organics, without water, and seemingly with compounds all around that rapidly bound other elements to oxygen and made them inaccessible to potential life. A consensus quickly formed that the reactions in Levin’s experiment and the others had to be chemical and not biological, and that’s the way the Viking results were presented to the world and understood by the scientists—all except for Levin and a handful of others, that is.
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The fact that at both Viking sites radioactive carbon dioxide appeared in significant amounts during his experiment and didn’t appear during the controls, that the experiments met all the criteria set out before launch for a positive finding of biological activity and life, was too much for Levin to leave undefended and let go. And so for more than thirty years he has done just that—reminding one and all about the Labeled Release results, citing tests of his experiment in extreme environments around the world, and working hard to knock down all the alternate explanations offered. Others have joined the fray in recent years, and Biemann’s mass spectrometer has been found wanting in a number of reviews by respected scientists. Those men and women don’t necessarily endorse Levin and his conclusions, but their research found numerous instances where the GCMS instrument would (in theory) and did (during testing) miss the presence of certain organic compounds in extreme Earth environments, especially when their concentrations were low. A 2010 paper by two prominent astrobiologists, Rafael Navarro-González of the National Autonomous University of Mexico and Chris McKay of NASA’s Ames Research Center, went further: They concluded the GCMS actually destroyed organics by heating them. And the chlorine-based organics that Viking scientists wrote off as trace contaminants from Earth were precisely what would be left behind if Martian organic material were heated along with surrounding Martian soil.
Even Biemann, who defends his Mars work vigorously as having determined that the Viking landing sites could not and did not support life, nonetheless does not believe it represents a final word on Martian biology. He ended a recent defense by writing: “Future missions to Mars will sooner or later answer the question of organic matter at the surface or in the near subsurface of that planet. It will require carefully designed instrumentation to carry out well planned experiments and thoughtful interpretation of the resulting data.” The implication, it would certainly seem, was that Viking did not meet that grade. The next NASA mission to search for Martian organics, the Mars Science Laboratory, will launch in 2011 and has a similar if more highly evolved GCMS that can test for organics (and unofficially for signs of life) using solvents rather than heat.
Levin, born in 1924, is now an adjunct professor at Arizona State University and has long run a firm based outside Washington, D.C., that discovered and developed a low-calorie sugar called tagatose now in final clinical trial as a diabetes drug. Behind his gentlemanly demeanor, he is a scientific warrior. When the principal investigator of the 2008 Phoenix mission to Mars told a TV interviewer that the lander was the first to touch frozen water on Mars but that the planet has no liquid water, Levin had a rejoinder on the show’s website within six minutes. “What a comedy!” he wrote. “Liquid water was discovered on Mars by the Viking lander in 1976! Ice was shown in images taken by the lander. We have published several papers proving liquid water on Mars. AND we claim that our Viking Labeled Release experiment detected living microorganisms on Mars…. Paradigm shifts are difficult, but this one has taken way too long!”
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Levin had just moved into a modest town house outside Washington when we first met, and many of his various scientific trophies and memorabilia were still in boxes. Although he has three degrees, including a doctorate from Johns Hopkins University, he believes that his beginnings in the world of sanitary engineering and that his Mars research was not done at a prestigious university are held against him.
“I’ve thought long and hard about this and I think that when the Viking results came in, NASA was confronted with evidence of life and no evidence of organics. One result came from a prominent professor and another from an unknown guy from a small company. The more conservative folks were more comfortable with Biemann and his ‘no organics,’ and that was the ballgame.” Nobody seriously questioned Biemann’s instrument until years later, when it was shown by several teams that the instrument could not detect very low levels of organic material in samples from Earth known by other means to have living microbes in them. By then decades had passed, and NASA was stuck with its no-life position because overturning it would raise a new set of other controversies. Levin’s conclusion: “Nobody in charge was brave enough to say it was wrong and NASA still doesn’t want to go near the issue. After Viking and until the present day, there have not been any life-detection experiments sent to Mars, even though finding life there would be the biggest discovery in the history of science.”
Not surprisingly, many see the Viking results and subsequent scientific approaches to Mars quite differently. For instance, Michael Meyer, the lead scientist for NASA’s Mars program, and who has a longtime involvement with astrobiology, said an essential lesson of the Viking missions was that we don’t really know how to look for life yet, an embrace, of sorts, of the Cleland and Ellington position. Levin’s experiment focused on an undisputed signature of life—metabolism—but Meyer says the results were positive but ultimately not convincing. “He might have found life,” Meyer said, “or he might have found that nonbiological processes take place on Mars very differently than they do on Earth.” The release of CO2 could have been the result of a not-yet-understood chemical reaction, for instance, if compounds with a lot of free oxygen were present. In other words, what would be a clear indication of biology and metabolism on Earth could be totally nonbiological on Mars. The upshot of the Viking life-on-Mars debate has been that NASA has studiously avoided sending life-detection experiments to the planet ever since, choosing instead to concentrate on geology, mineralogy, weather, and the search for water present and past.
In the early 2000s, the United Kingdom sent Beagle 2, a small probe designed to look for life-sustaining habitats, to Mars, and Levin tried without success to get a life-detection instrument into the mix there as well. Speaking with BBC News before the planned landing, deputy mission manager Mark Adler explained that Beagle’s mission was to better understand the water environment of Mars and not to search for life as Levin urged. “What we learnt from Viking is that it is very difficult to come up with specific experiments to look for something when you don’t really know what to look for.” But it all became moot when Beagle’s mission control lost contact with the spacecraft as it entered the Martian atmosphere, and disappeared. Levin did succeed in getting a bare-bones life-detection experiment onto a Russian mission to Mars in 1996, but that effort failed before it even reached the planet.
Still Levin is seeking vindication and has (among others) his physicist son Ron Levin working with him. In 1986, the senior Levin told a Viking ten-year reunion gathering at the National Academy of Sciences that “it is more likely than not that the Viking LR detected life.” In 1997, he argued in a paper for the Proceedings of the International Society for Optical Engineering, which society has an active astrobiology program, that twenty years of additional Mars research had convinced him that his Viking experiment had definitely detected life and that NASA and the scientific consensus were wrong. Nine years after that publication, with an appreciation of Levin’s work emanating from a new generation of Mars scientists, an Argentinian scientist proposed the name Gillevinia straata as the genus and species of the bacteria-like organism ostensibly identified by Viking. But that idea did not garner much support. Levin was not invited to give a talk at the official thirtieth anniversary of the Viking mission.
To this day, Levin is not inclined to think that the absence of a firm definition of “life” played a significant role in the scientific community’s reluctance to accept his Viking data; it’s something of a red herring, he says, used to protect important people from having to admit they were wrong. But that refusal to entertain other possibilities, to essentially reject the notion that testing for life on Mars might require a different way of thinking, is what frustrates many scientists about Levin. Yes, Levin is tirelessly and heroically defending a result defined at the time as positive. Yes, the experiment uncovered something of great interest, and nobody has been able to explain why the Labeled Release and its control behaved as they did.
But thirty years of additional research and thinking about Mars has, in many ways, turned Viking’s simple models about life on their heads. As Meyer explained it, “In some ways, you could say that Viking was too Earth-centric. It presumed life has metabolism and respiration that results in production of carbon dioxide that we can recognize. It also presumed that if you land anywhere on Mars you can measure life.” He said that while NASA has at times used the definition of life as a “self-sustained chemical system capable of undergoing Darwinian evolution,” it was not a formal position, and the agency was increasingly inclined to accept the reasoning of Cleland and others that “life” cannot be currently defined, any more than water could be in the sixteenth century. “Probably the characterization people are most comfortable with is the Supreme Court one on pornography, that ‘we know it when we see it.’ But for a variety of pretty obvious reasons, that one really didn’t fly.” Describing essential characteristics of life—that’s certainly possible. But a final, all-encompassing definition that provides the invaluable solid ground scientists have been searching for, that will have to wait.
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As practitioners of another arm of astrobiology will quickly point out, you don’t have to go to Mars to get confused about whether something is alive—that is, the result of biological processes—or not. A parallel and sometimes equally intense debate has been going on for several decades about a substance found on Earth called desert varnish. Best known as the purple-black background to many ancient American Indian drawings (or petroglyphs) in the southwestern United States, the varnish coats rocks in arid climates. Nobody really knows how it gets there. Researchers have found large colonies of bacteria living beneath the very thin yet definitely layered varnish, and they have found very high concentrations of the element manganese in the coverings as well.
Living things have the property of concentrating elements in ways that nonbiological processes do not, and so the unusually high levels of manganese and sometimes iron in varnish—much higher levels than in the surrounding environment—definitely suggest biology. The opposing line of thought is that the bacteria found under the varnish have come from elsewhere and have simply found a protected place to live in very harsh environments. As for the high concentrations of those elements not concentrated in the soil or other rocks nearby, those are the result of chemical reactions and collections of windblown dust. You would think this would be a relatively simple question to answer, but it isn’t. Desert varnish thickens at an extremely slow pace, on the order of between 1 and 40 micrometers (or 0.000003937 to 0.00015748 inches) per every thousand years, so it has been impossible to experiment definitively with it in the lab. After centuries of growth, a rock’s varnish covering will be about the thickness of a piece of paper. And nobody knows how or why it spreads.
As is often the case in astrobiology, the players in the desert varnish story come from a broad range of backgrounds—specialists in caves, in planetary science, in geography, in engineering. They get pulled in, not only by a desire to unravel the mystery of the origins and nature of desert varnish, but also because of some images of rocks that have come in over the years that seem to show something that looks surprisingly like desert varnish in an unexpected location: Mars. Both NASA and the National Science Foundation have funded research into desert varnish, and some years ago it was a very hot topic. The combination of painfully slow progress in understanding how the varnish grows, along with a mini-scandal in the field regarding some questionable data, has pushed it to a back burner. But that doesn’t mean some intrepid souls are not still surveying the murky borderland between biological and nonbiological life.
One is Penelope Boston of the New Mexico Institute of Mining and Technology in Socorro. She is an expert in caves and the microbes that live in them, but also has a passion about both Mars and desert varnish. A woman of many enthusiasms—her department office is overflowing with alien action figures, stuffed animal bats, robots, and name tags from hundreds of conferences around the world on caving, Mars, and astrobiology—she is happiest out as a field researcher. It was while doing a five-day research expedition in the Lechuguilla limestone cave in the Carlsbad Caverns National Park, the deepest cave in the nation, that desert varnish came into her life. She was quite deep in the cave when some “fluffy” greenish-reddish-purplish material swirling in the air fell into her eye. It didn’t take long for her eye to swell shut, leading to a harrowing rope climb up and out of the cave but—more important to her—also yielding one of those “aha!” moments when it becomes clear things are not what they seem.
A trained microbiologist, Boston immediately understood that some microbes had gotten into her eye, which meant that they were living deep below the Earth’s surface on what appeared to be rock face. This was before Onstott’s South Africa work, so the scientific consensus was that nothing was alive in a deep cave, especially one known to be virtually locked off from the surface. Once she got outside, the swelling disappeared within four hours because, she also surmised, the microbe could not survive in the light of day. She and her colleague Chris McKay, of NASA’s Ames Research Center, returned and over several years concluded that the fluffies were coming from the manganese and iron deposits in the caves, and that they were all part of a living microbial world. Driving around New Mexico, Boston constantly passed rock varnishes that featured the same manganese and iron that produced whatever had landed in her eye, and she got to thinking about what microbes might also be living in the varnish, or perhaps forming the varnish.
This encounter led to more than ten years of collecting samples of desert varnish and culturing them in her lab. The result is now a room filled with hundreds of stacked petri dishes, cylinders, and plates—some being warmed in incubators and some refrigerated—alive with what she is convinced are the bacteria responsible to a greater or lesser degree for the presence of desert varnish. The bacteria come from her home state, from Mexico, from Utah, from Chile, from volcanoes, from extreme environments of all kinds, and all are now growing in some agar medium and usually producing in bountiful quantities the purple-black signature of manganese. All started in a clear medium with tiny scrapings of bacteria added from a desert varnish sample, and most had produced massive (on desert varnish scales) amounts of the manganese-concentrating bacteria over the years. Surrounded by so much life, however microbial it might be, Boston talks to the samples, refers to them as “these guys” or “those guys,” and says “we’ve got everyone in here.” She knows which are “going gangbusters” and which are struggling to survive. One she describes as resembling, under heavy magnification, a bundle of grapes. “Look at this one,” she says, pointing to a seemingly dead, crusted collection in a vial. “They look dead, but they’re accustomed to desert conditions so they adapt. Rehydrate them a bit and they’ll be growing fast, too.”
Has Boston proven the desert varnish is indeed a product of living organisms? Not really. As she readily acknowledges, growing the samples demonstrates that the bacteria can and do concentrate manganese as contained in desert varnish, but that lab process has limitations. To achieve a level of proof, she would have to place her samples on rocks and see how they grow, and there’s the rub: They grow at such a painfully slow rate that no professor, no graduate students would still be around to detect their progress. In Death Valley, it takes varnish something like ten thousand years to grow to a thickness of one-hundredth of an inch. She has yet to settle on a plan, but she has elaborate schemes for trying to force that growth in field conditions. That’s probably to be expected from a woman who once lived two weeks in a simulated Martian environment in Utah and was one of the founders in the early 1980s of what became known as the Mars Underground at the University of Colorado, Boulder. A group of students who were fascinated by the planet and wanted, in the wake of the Viking disappointments, to keep interest in it alive, they sponsored a series of conferences that attracted prominent scientists and some NASA officials. Boston dreams of being around when life is discovered there; actually, she said she would gladly fly on the maiden yearlong voyage to Mars. In the meantime, as she tries to unravel through field and lab work the barest-bones life on Earth, she is getting help from another scientific outlier.
Tom Nickles has also been lured into the borderlands of desert varnish, and he is now conducting the experiment he believes will finally determine whether living bacteria or nonliving chemicals are responsible for the coverings. In a world of unusually bright people with unusual backgrounds, Nickles perhaps takes the cake. Tall and thin, he has wavy hair that served him well during his days as an occasional Elvis impersonator. He wears a belt with a large buffalo head buckle, which fits both his name and his locale—the University of Idaho in Moscow, Idaho. A trained engineer, former air force intelligence analyst in Turkey, test pilot trainer at Edwards Air Force Base in his beloved Mojave Desert (where he ran marathons), and so much more, he went back to school at age fifty to get a doctorate in astrobiology. There was no astrobiology program at the University of Idaho, but he found a professor who would sponsor him and he’s now several years into the program. But he’s hardly your typical doctoral student: He gets called in to do consulting for NASA and his plans for the desert varnish experiment looked so promising that he was invited to speak to the American Chemical Society’s astrobiology panel before he even began the work.
In a stainless steel glove box with Plexiglas linings, floors, and dividers the size of a blanket chest, he had created an environment similar to the Mojave—with fans to simulate the wind, special lights to simulate the desert glare, and some extra moisture to speed up the growth process and simulate six years of varnish development in one. He divided the box in half, but both halves had a bed of sandy soil basalt and quartz as anchor for the varnish, should it grow. The two were identical except that on one side he planned to introduce some bacteria he had collected from varnish found outside Baker, California, at the Lima Lava Flow. The other side would have none of those bacteria. Would either, or both, lay down a varnish?
“Both sides start with absolutely nothing alive. The chambers have been sterilized and the substrate—the quartz and agate and basalt—have been autoclaved, so I’m sure everything will be dead. I will seal the abiotic side tight so absolutely nothing gets in, but on the other side I’ll paint some of the bacteria I collected from varnish onto the rocks. Then I wait and watch. The fans will blow the dirt and dust around and the UV light will shine and the moisture will seep in and it will be just like the desert, except speeded up a bit.”
The experiment hadn’t yet begun when I visited; the varnished (biotic) samples were placed in the box a few months later. Nickles didn’t expect to see black rock varnish anytime soon; that takes way too long. But the bacteria, with their extra UV light and moisture, could begin the unseen varnish-making process in months, or maybe a year. That process involves one of the most important dynamics of both astrobiology and geology: Life forms interact with minerals and rocks, and transform them in minute but detectable ways. They impose a distinctly biological structure onto their nonliving surroundings, and they also can (and usually will) concentrate certain elements or minerals in the process. In the case of desert varnish, the coating is blackened by a concentrating of manganese, or can come out reddish brown if the bacteria is concentrating iron.
“The experiment will go for a year, but I’ll first open up the biotic side at three months. I’ll take out three specimens at random and then will put them in the antechamber,” an airtight but accessible cylinder outside the glove box. “After that comes the electron microscope to see if anything is being moved around. I’m looking for just a very crude laying down of structure, of organization, and the start of some layering. If biology plays a role in making varnish, we should start seeing [very early signs of activity] on the rock. Nothing beautiful like nice, glossy varnish forming. But something with structure.”
Or maybe not. Maybe a varnishlike coating will emerge on the side without varnish bacteria instead, giving support to the theory that the varnish is formed by chemical processes involving the wind, the sun, dust, and the surface of the rock. And by implication, any varnishlike formations on Mars would have the same nonbiological origins. All of this raises the same fundamental question: If we can’t determine or define what is life on Earth, how can we possibly do it on Mars or Europa or anywhere else?
On Earth, we at least know the basic molecular, chemical, and thermodynamic outlines of carbon-based life, but we are still in something of a quandary when it comes to definitively nailing it down. That’s why scientists generally focus now on the known effects of living things on rocks, water, and atmospheres. We may not know what life is, but we have a pretty good idea of what life does—or at least the kind of carbon-based life found on Earth. What if extraterrestrial life is silicon based or has a very different way of holding and transmitting the information that produces future generations? The National Academy of Sciences formed a panel to study what came to be known as “weird life.” It met periodically from 2002 to 2005 and released a report in 2007. Not surprisingly, it complicated rather than clarified the question of what life is by offering possibilities based on silicon (instead of carbon) and replacing water as the key solvent with ammonia or methane. Science has no examples of such life, but silicon has bonding properties similar to but less adaptable than those of carbon, and life based on a solvent other than water is also considered theoretically possible by some.
In science, the most desirable and convincing proof of a finding or theory is to replicate the reaction reliably under controlled settings. So some scientists are trying to make life out of nonliving elements and compounds in their labs. It’s not exactly Dr. Frankenstein redux, but close. If they can create from component parts an entity that replicates, that takes in and uses energy, and that is able to both mutate and repeat that mutation with its replicator, then they can lay claim to having achieved a proof of concept. The creation wouldn’t tell us how life actually began, but it would represent a process through which life could have begun. And along the way, it will help define, or at least to better characterize, what life actually entails.
It’s a definite competition among some twenty of the world’s most innovative and admired labs, but because the task is so daunting, it is also collaborative. But none of the scientists involved is as dazzling or as excitedly eclectic in their work as Steven Benner, a chemist and molecular biologist who created and runs the nonprofit Foundation for Applied Molecular Evolution in Gainesville, Florida. He keeps afloat on grants from NASA and the NSF, but also and most importantly on the profits from the creation of the world’s first synthetic genetic system capable of producing unnatural nucleotides (the parts of DNA and RNA involved in pairing) used to monitor the levels of viruses that range from HIV to hepatitis B. With that support, he is able to push forward with efforts to produce that “self-replicating chemical system capable of Darwinian evolution” that defines life in its most broadly accepted form. With his wide-ranging knowledge and willingness to look seriously at problems from new, untried angles, he’s often called on to help NASA and the National Academies of Science tackle big, complex issues; most recently, Benner was one of a handful of scientists asked by the National Academy to study the possible biochemistry of extraterrestrial life, the effort that produced the “weird life” report.
Benner’s Gainesville lair is hardly the highly organized, precision-driven lab you might expect. Certainly the area where his almost twenty-person team of chemists and biochemists do their molecular slicing and dicing is well controlled, and the work of experts in paleogenomics (who read the evolutionary history of life-forms through their genomes) requires mind-numbing precision. But the heart of the operation, where the sparks fly, is Benner’s office. At its center is a fifty-two-inch Sharp Aquos computer screen connected to a smaller one—the blackboard on which he diagrams chemical systems. He goes at the task with the focus and energy of an artist captured by a moment of creativity, and the screen can soon fill up with hundreds of connected C’s (carbon) and H’s (hydrogen) and P’s (phosphorus) as they interact and loop around to form known or possibly synthesized biochemical cycles. Benner, talking and writing nonstop, sits in an oversize pinkish reclining chair, taped in the back where the material is cracked. On prominent display around him is a sampling of his collection of minerals and fossils of fish and plants, ferns and small mammals. Benner has been a fan of both rocks and fossils since he was a boy, and each object has a story.
The boron-based rock on his shelf, for instance, captures one of his scientific eureka moments. Benner had been on Catalina Island, off Los Angeles, with a group of geologists, and he was leading them through some experiments involving the sugar ribose, a mineral with calcium in it, and water. The goal was to find a way to keep the combination from turning into brown tar, which seldom has anything useful in it from a biological perspective. This is a significant issue in the origins-of-life world because ribose is the R of RNA, and it has to be stable enough at some point to bond with the other elements. (Stanley Miller, of the Scripps Research Institute, outside San Diego—the deceased godfather of origins of life experimentation—famously found some precursors to the building blocks of life in 1954, opening the door to what was assumed to be a fairly imminent test-tube creation of life from nonlife. But it never happened and in 1995 Miller basically said it couldn’t—primarily because ribose is unstable in the water that is assumed necessary to support life.) Benner long knew that the element boron at least temporarily blocked the decomposition of ribose in water, but he had never before thought to throw it into the origin-of-life chemical mix. But he did in Catalina, and the ribose did not immediately start the usual quick slide into tar when a pinch of boron was added. Instead it stayed clear, and a very excited Benner believed he had found a way to allow the essential ribose to be created on early Earth while still keeping water in the picture. This epiphany didn’t solve the question of how life formed from nonlife, but it offered a plausible explanation for how one of many obstacles may have been overcome. When I first heard Benner speak at an astrobiology conference, he made a quick but quite serious aside suggesting that boron really could be central to the creation of life—even though it is one of the less common elements on Earth and across the universe.
The origins-of-life world has two competing schools. One says that the Last Universal Common Ancestor (or LUCA) formed as “life” when genetic material came together and self-replication could begin. The other view is that metabolism—the process of taking in the energy of food, using it, and then expelling a waste product—was the essential first step. Benner is in the genetics camp, which is why the origins-of-life component of his lab spends its time searching for ways to form the scaffolding of RNA or DNA out of nonliving parts. So many confounding factors are involved that biochemists like Benner are among the most skeptical about finding life beyond Earth. From the perspective of physicists, astronomers, biologists, and others in the astrobiology world, extraterrestrial life is a given. But to the chemists and biochemists working to actually get life started, the prospects are not as tangible. Making life from nonlife has turned out to be extraordinarily hard.
Benner says he assumes the actual origin of life—the pathway that created the first organisms that could feed, could replicate themselves, and then could evolve by producing mutant replications and keeping those that turned out to be useful—will remain unknowable. But finding another way that nonlife turned into life would be an entirely acceptable, actually enormous historical triumph, because it would provide the indispensable “proof of principle” that cutting-edge scientists are always looking for.
“Look, we know that life evolved from nonliving sources because otherwise we’re left with divine intervention—which is hardly an acceptable explanation for most scientists. That’s what keeps some of us going. It will be enormously difficult to find how it happened, but we know it did happen, either on Earth or elsewhere and then transplanted here.”
The field has seen progress, even if it hasn’t had the big breakthrough that many are looking for. In 2009, the journal Science published the results of work in the Scripps lab of Gerald Joyce, who with Tracey Lincoln produced an RNA enzyme (a protein that increases the rate of chemical reactions) that was a super replicator capable of building copies of itself over and over again, something never done before with RNA. This high-powered enzyme, refined and concentrated through cultures, met the primary goal of being able to perpetually replicate itself, as well as to mutate and then pass on the genetic blueprint of that mutation to other RNA. “This is the only case outside biology where molecular information is being passed through the generations [and] has become immortal,” is how Joyce put it.
Joyce is now working to expand the functions of that immortal replicator and to see if it could survive and prosper in a more varied and complex system, if the stronger enzymes created by the lab could be challenged to invent new functions, just as RNA material presumably had to do on the early Earth. If he succeeds, he said, the lab will have indeed created life. “We believe genetic material that can respond to increasingly complex challenges represents life.” I later asked Benner if that would meet his criteria, and he gave a quick “yes” as well.
But synthetic biology has an Achilles heel, hidden in plain sight: All of the researchers in the field make their creations using strands of preformed DNA and RNA that they can buy from a supply house. Manipulating those strands to make something out of them that can copy itself and somehow get energy from its environment would be an enormous achievement, but it would still require the scaffolding of genetic material to be delivered by overnight mail in a vial. Clearly, that’s not how life started. Producing “life” in a lab may soon turn out to be possible, but it would require not only a lot of already formed complex molecules but also a lab full of scientists and equipment. Synthetic biology is science at its most imaginative and sophisticated, but at bottom it can’t exist without the kind of intelligent designer that would give comfort to those who subscribe to a religious view of the origins of life—it requires a Creator. So the goal is a proof of concept, not a re-creation of the origins of life. Matt Carrigan, an origins-of-life researcher in Benner’s lab, talks of research that would “jump the chasm,” that would put together unprocessed molecules—not from the supply store—in a way that would allow them to begin life. But that kind of biochemistry seems very far in the future.
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Ironically, it is the search for life beyond Earth, rather than the quest to synthesize it in a lab, that may hold the greatest promise for determining what constitutes life. On Earth, we have one essential model of life: Every living thing takes in energy and expels waste, maintains a thermodynamic balance, and, most remarkably, uses the same twenty amino acids to form the proteins that do all the heavy lifting within cells. What’s more, all life on Earth uses the nucleotides adenosine triphosphate (ATP) and adenosine diphosphate (ADP) to store and distribute energy within cells. It’s been extraordinarily difficult—somewhat like imagining a new color in the spectrum—to put together any respectable theories of what a significantly different extraterrestrial life might look like and how it might work. The National Academy report on “weird life” put it succinctly: “As Carl Sagan noted, it is not surprising that carbon-based organisms breathing oxygen and composed of 60 percent water would conclude that life must be based on carbon and water and metabolize free oxygen.”
So any discovery of a life-form not descended from LUCA would provide this remarkable bonus: Finding Extraterrestrial Life 2.0 would make far more clear what Earthly Life 1.0 actually entails, and what is needed for something to be alive. It’s a head-spinning conclusion, counterintuitive in the extreme. But the discovery of extraterrestrial life, of life as we don’t know it, may be the only way to finally define what actually constitutes life on Earth.