Genesis: The Scientific Quest for Life's Origin

by Robert M. Hazen

The first few data points seemed to support the hypothesis. The Murchison and Allan Hills meteorites showed phenanthrene-to-anthracene ratios of 1.7:1 and 2:1, respectively. The biogenic Burgess Shale and a mature coal, on the other hand, yielded much higher ratios, close to 15:1. But then results began to scatter. Some low-grade coals had ratios less than 5:1, while the black, fossil-rich Enspel Shale was only about 2:1. The promising hypothesis began to crumble.

  After several weeks of effort, and a thorough review of the published literature, Rachel discovered that we were simply reinventing the wheel. Coal experts have long known that anthracene is slightly less stable than phenanthrene. Consequently, “high-grade” coals that have experienced prolonged high temperatures and pressures have a higher ratio of phenanthrene to anthracene, topping 20:1 in some specimens. Such a high ratio in any sample may have resulted from prolonged heating and have nothing at all to do with a biogenic past.

  Our only useful conclusion was that unambiguously biological specimens never seem to display ratios less than about 2. So a lower ratio of phenanthrene to anthracene, as found in meteorites, synthetic-run products, and soot from burning carbon, may provide a valid abiomarker.

  The next step? Perhaps we'll try another PAH ratio, such as that of phenanthrene to pyrene, a particularly stable diamond-shaped molecule with four interlocking rings (C16H10). One thing is certain: we'll never run out of molecules to try.

  LOOKING FOR LIFE ON MARS

  The most exciting, important, and potentially accessible field area to look for ancient alien life is Mars, which, like Earth, formed some 4.5 billion years ago. A mass of new data points to an abundance of surface water during the planet's first billion years, dubbed the Noachian epoch by Mars geologists. Sunlit lakes, hydrothermal volcanic systems, and a benign temperature and atmosphere might have sparked life and made Mars habitable long before Earth. Perhaps fossils, molecular and otherwise, litter the surface.

  The quest for Martian life has a checkered history. A century ago, American astronomer Percival Lowell reported observations of a network of canals on the red planet—evidence of an advanced civilization, he thought. Such speculation fueled the imaginations of science fiction writers, but hardened the scientific community to such unsubstantiated claims. NASA's remarkable Viking Mars lander of the mid-1970s carried an array of experiments designed to find organic compounds and to detect cellular activity, but ambiguous results merely led to more controversy. Hot debates over purported fossils in the Allan Hills Martian meteorite represent just one more chapter in this contentious saga. Given such a troubled context, NASA will choose its next round of life detection experiments with the greatest of care.

  Humans aren't going to set foot on Mars anytime soon, but that's what NASA's amazing rovers are for. Sojourner, Spirit, Opportunity, and other robotic vehicles provide the experimental platform; they haul the instruments across the desolate Martian surface to probe tantalizing rocks and soils. The key to finding life (or the lack thereof) is to design and build a flight-worthy chemical analyzer for life. That has been the occupation of Andrew Steele throughout much of his career.

  “Steelie,” my ebullient colleague at the Geophysical Laboratory, is a microbial ecologist who got his start studying the microbial corrosion of stainless steel in nuclear reactors. British Nuclear Fuels Ltd. sponsored his PhD thesis, which is still largely classified and unpublished. Radioactive isotopes often contaminate the thin outer layer of stainless steel that lines nuclear reactors—a difficult and costly cleanup problem. Steelie invented new microscopic techniques to study the steel surfaces, and he found that some microbes secrete biofilms that rapidly eat away steel, thus stripping off the affected layers and greatly simplifying decontamination. [Plate 4]

  In 1996, just two weeks after defending his PhD thesis in England, the Allan Hills meteorite story broke. The timing was perfect, and Steelie was hooked. Setting his sights on nonradioactive ecosystems, he became obsessed with attempts to detect ancient life from the faint molecular traces in rocks. He contacted the NASA team, who were lacking in microbiology expertise and thus eager for his help. David McKay sent him a sample of the precious Martian rock, and within a year Steele had applied his microscopic techniques to investigations of the purported microbes. He made a memorable presentation at NASA's annual Lunar and Planetary Science Conference in Houston and landed himself a job as a NASA microbiologist working at the Johnson Space Center under McKay's guidance.

  Steelie's assignment at NASA was to head the JSC Blue Team (consisting of Steele and a fellow gadfly), who were to try out every possible idea to disprove the hypothesis of Martian life in the Allan Hills meteorite. McKay led the significantly larger competing Red Team. McKay's strategy of examining both sides of the issue was laudable and in the best tradition of scientific objectivity, but it may have backfired when Steelie did his job too well. He performed a series of high-resolution microscope studies and by 1998 had discovered that the Allan Hills specimen (like virtually every other meteorite he had ever examined) was riddled with contaminating Earth microbes. McKay was unconvinced and continued to promote his original interpretation of ALH84001. Discouraged at the cool reception accorded his findings, and missing his wife and family in England, Steele left his steady NASA employment in early 1999 for a hectic schedule of visiting professorships and research jobs at the Universities of Montana, Oxford, and Portsmouth, along with more NASA consulting.

  About the time that Andrew Steele's research was making him something of a persona non grata at the Johnson Space Center, the Carnegie Institution's Geophysical Laboratory found itself under new leadership. Wes Huntress, the former associate administrator of the NASA Office of Space Science (and the man who introduced the term “astrobiology” to the NASA community), had been hired to shake things up and expand Carnegie's fledgling astrobiology effort. In a bold and welcome move, Huntress made Steele his first staff appointment in 2001. Steelie's peripatetic scientific lifestyle seemed unsuited to the traditional academic world, but his unconventional background was just the ticket for the Geophysical Lab.

  He arrived like a whirlwind, his shoulder-length blond hair and open tie-dyed lab coat flying behind him as he dashed between office and lab. Crates and boxes arrived by the dozen at his second-floor domain, two doors down from my own office. He crammed his lab space with DNA sequencers, chip writers and chip readers (benchtop machines that prepare and read slides), and a bewildering array of other microbiological hardware, most of which none of us geologists had ever seen before. A small army of bustling postdocs followed and the corridor took on new life. Chemist Mark Friese brought his collection of orchids to grace one alcove with a forest of exotic blooms. Molecular biologist Jake Maule maintained his pro-level golf game by challenging all comers to putting contests—a regular stream of golf balls began rolling past my open doorway.

  In such an environment, new ideas fly thick and fast. Steelie had a master plan. He wanted to build a life-detecting machine to fly to Mars—a huge interdisciplinary project, given how much we still don't know. On the scientific side, he had to figure out what constitutes a legitimate biosignature, so Steelie embarked on various paleontology projects, principally with German postdoc Jan Toporski, who happens to be his brother-in-law and soccer buddy. (There are also a lot of soccer balls and other soccer paraphernalia in the corridor.) They focused on a 25-million-year-old fossil lake in Enspel, Germany, where well-preserved fish, tadpoles, and other animals are found. Studies of their molecular preservation would provide important hints about life's most stable and diagnostic molecular markers.

  Then there was the detection part. It was essential to develop an unambiguous procedure to find the tiniest amounts of any target molecule. That's where Jake Maule came in. An aspiring astronaut trained in clinical medical technology, Jake's job was to develop molecular antibodies—proteins with specialized shapes that would lock onto only one type of target molecule. [Plate 4]

  Jake focused on producing hopane antibodies for us
e in a rapid and sensitive field test for microbes. His procedure involved injecting mice with hopanes and letting their immune systems do most of the work. Hopanes are too small to evoke an immune response by themselves, so Jake attached hopane molecules to a big protein called BSA (for bovine serum albumin). He injected 30 mice with a hopane–BSA solution, waited a week, and gave each a booster shot. In about three weeks, the mice manufactured a suite of hopane-sensitive antibodies. From each mouse, Jake extracted a syringe full of blood, which was centrifuged to separate out the red and white blood cells from the watery fluid that held the antibodies. Ultimately, each mouse yielded one tiny, precious droplet of that antibody-rich fluid.

  Armed with hopane antibodies, Maule was ready to analyze ancient rocks, in an elegant four-step process.

  Step 1: He crushed various promising rock samples and washed them in a solvent, which concentrated any hopane residues. Those solutions were loaded into the chip writer, a sleek benchtop machine about the size of a breadbox that placed an array of tiny dots of the solutions (some containing hopane and others not) and various standards onto a glass slide.

  Step 2: With the chip writer, he applied a tiny amount of hopane antibody onto each of the sample spots, then rinsed. Some spots then retained hopanes with attached antibodies, while other spots were washed clean.

  Step 3: He then treated each spot with another solution, this one containing a second antibody that locks onto any mouse antibody and is also highly fluorescent. All the spots with hopanes attached to mouse antibodies would thus fluoresce.

  Step 4: Finally, he took the glass slide with its array of spots and put it into a second sleek machine, the chip reader, which, as its name suggests, recorded which spots fluoresced and which spots didn't. Like microscopic lightbulbs, his antibodies glowed when hopanes were present in the rock sample.

  Jake had devised a fast, automated process for identifying hopanes. But all that work and more was mere prelude to the most crucial aspect of a flight-ready analytical instrument—the design and engineering. Concepts and benchtop demonstrations were one thing, but an instrument on Mars has to be absolutely reliable, shock resistant, and very, very small. Steelie began dealing with nitty-gritty questions of how to collect a soil sample, how to introduce a small amount of sterile solvent to dissolve the target molecules, how to excite a fluorescent signal, and how to relay the information home to Earth, all in a tiny box. Soon, armed with a million dollars of NASA funding, he planned to fly the instrument on NASA's 2013 mission to Mars.

  Steelie's baby is called MASSE—the Microarray Assay for Solar System Exploration. Adapting the latest in chip writer/chip reader technology, his team is building both flight-ready and handheld devices that use antibodies to detect trace amounts of dozens of diagnostic biomolecules: hopanes, sterols, DNA, amino acids, a variety of proteins, even rocket exhaust. Carnegie is not alone in developing such an instrument, and given the intense competition of other dedicated design teams, there's no guarantee that MASSE will ever fly. Steelie is also competing against a seductive sample-return mission—a technically challenging effort to return a soda can-sized canister of Martian rock and soil to Earth on the same 2013 mission. Only one instrument package will be selected. Even if MASSE does fly, there's no guarantee that it will arrive safely, and if it does arrive safely that it will find anything of interest. Of course, Steelie and his group are learning lots of fascinating stuff along the way. And I've never seen scientists have so much fun.

  In our quest to understand life's emergence, fossils provide essential clues. Even lacking morphological evidence, fossil elements, isotopes, and molecules point to the nature of primitive biochemical processes. These microscopic remains also reveal a diversity of life-supporting environments and help to constrain the timing of life's genesis. What's more, if we're lucky, fossils from Mars or some other extraterrestrial body may eventually provide the best evidence that life has emerged more than once in the universe.

  And yet, valuable as these insights may be, all known fossils represent remains of advanced cellular organisms similar to those alive today. Few, if any, clues remain regarding the emergent biochemical steps that must have preceded cells. To understand how life arose, therefore, we must go back to the beginning and approach the question of origins from the bottom up.

  Interlude—God in the Gaps

  Darwinists rarely mention the whale because it presents them with one of their most insoluble problems. They believe that somehow a whale must have evolved from an ordinary land-dwelling animal, which took to the sea and lost its legs.

  …. A land mammal that was in the process of becoming a whale would fall between two stools—it would not be fitted for life on land or sea, and would have no hope of survival.

  Alan Haywood, 1985

  This rant by Alan Haywood, and similar silly statements by his creationist colleagues, reveals a deep mistrust of the scientific description of life's emergence and evolution. But he is correct in one sense. Science is a slave to rigorous logic and inexorable continuity of argument. If life has changed over time, evolving from a single common ancestor to today's biological diversity, then many specific predictions about intermediate life-forms must follow. For example, transitional forms between land mammals and whales must have existed sometime in the past. According to the creationists in the mid-1980s, the lack of such distinctive forms stood as an embarrassing, indeed glaring, proof of evolution's failure. Their conclusion: God, not Darwinian evolution, must have bridged the gap between land and marine animals. But what appeared to them as an embarrassment for science then, has since underscored the power of the scientific method.

  Science differs from other ways of knowing because scientific reasoning leads to unambiguous, testable predictions. As Haywood so presciently predicted, whales with atrophied hind legs must have once swum in the seas. If Darwin is correct, then somewhere their fossils must lie buried. Furthermore, those strange creatures must have arisen during a relatively narrow interval of geological time, bounded by the era before the earliest known marine mammals (about 60 million years ago) and the appearance of streamlined whales of the present era (which appear in the fossil record during the past 30 million years). Armed with these predictions, several paleontologists plotted expeditions into the field and targeted their search on shallow marine formations from the crucial gap between 35 and 55 million years ago for new evidence in the fossil record. Sure enough, in the past decade paleontologists have excavated more than a dozen of these “missing links” in the development of the whale—curious creatures that sport combinations of anatomical features characteristic to both land and sea mammals.

  Moving back in time, one such intermediate form is the 35-million-year-old Basilosaurus—a sleek, powerful, toothed whale. This creature has been known for more than a century, but a recent discovery of an unusually complete specimen in Egypt for the first time included tiny, delicate vestigial hind leg bones. That's a feature without obvious function in the whale, but such atrophied legs provide a direct link to four-limbed ancestral land mammals.

  And then a more primitive whale, Rodhocetus, discovered in 1994 in Pakistani sediments about 46 million years old, has more exaggerated hind legs, not unlike those of a seal. And in that same year paleontologists reported the new genus Ambulocetus, the “walking whale.” This awkwardly beautiful 52-million-year-old creature represents a true intermediate between land and sea mammals.

  Nor does the story end there. In September 2001, the cover stories of both prestigious weekly magazines Science and Nature trumpeted the discovery of a new proto-whale species that had just been reported from rocks about 50 million years old. Nature's cover story, titled “When whales walked the earth,” underscores the power of science and the futility of the creationists' task. Science makes specific, testable predictions. Anyone can go out into the natural world and test those predictions. The creationists were wrong.

  Today's creationists have toted out a new version of this old “God-in-the-ga
ps” argument under the fancy name “intelligent design.” Their argument goes like this. Life is so incredibly complex and intricate that it must have been engineered by a higher being. No random natural process could possibly lead from nonlife to even the simplest cell, much less humans. The promoters, notably Michael Behe and William Dembski, don't talk about “God,” but they leave open the question of who designed the designers.

  The evolution of whales is illustrated by recent fossil finds, including Ambulocetus (52 million years old), Rodhocetus (46 million years old), and Basilosaurus (35 million years old) (from National Academy of Sciences, 1999).

  Such an argument is fatally flawed. For one thing, intelligent design ignores the power of emergence to transform natural systems without conscious intervention. We observe emergent complexity arising all around us, all the time. True, we don't yet know all the details of life's genesis story, but why resort to an unknowable alien intelligence when natural laws appear to be sufficient?

  I also see a deeper problem with intelligent design, which I believe trivializes God. Why do we have to invoke God every time we don't have a complete scientific explanation? I am unpersuaded by a God who must be called upon to fill in the gaps of our ignorance—between a cow and a whale, for example. The problem with this view is that as we learn more, the gaps narrow. As paleontologists continue to unearth new intermediate transitional forms, God's role is squeezed down to ever more trivial variations and inconsequential modifications.

  Isn't it more satisfying to believe in a God who created the whole shebang from the outset—a God of natural laws who stepped back and doesn't meddle in our affairs? In the beginning God set the entire magnificent fabric of the universe into motion. Atoms and stars and cells and consciousness emerged inexorably, as did the intellect to discover laws of nature through a natural process of self-awareness and discovery. In such a universe, scientific study provides a glimpse of creator as well as creation.