Genesis: The Scientific Quest for Life's Origin

by Robert M. Hazen

  Finally, the fifth point: ALH84001 holds myriad tiny sausage-shaped objects reminiscent of some species of terrestrial bacteria. Though much smaller than any known Earthly microbes, these suggestive forms provided the public with its most convincing evidence for Mars life. Hundreds of newspapers and magazines reproduced the NASA electron microscope images with captions identifying them as “Martian microbes.”

  The main text of the McKay et al. six-page article in Science conveyed a sober and reasoned discussion of their findings, and they acknowledged that no single line of evidence was enough to trumpet the discovery of alien life. But the concluding sentence shifted tone and pushed the limits of most readers' credibility: “Although there are alternative explanations for each of these phenomena taken individually, when they are considered collectively, particularly in view of their spatial association, we conclude that they are evidence for primitive life on early Mars.”

  To paraphrase the late Carl Sagan, extraordinary claims require extraordinary proof. Predictably, controversy exploded around the NASA scientists' bold claim. Experts pored over the paper, which was aggressively challenged on every point.

  Point number one: PAHs and other carbon molecules litter the cosmos, notably in the interstellar dust that forms comets and asteroids—the raw materials that formed Mars. What's more, such molecules would have formed in abundance by natural chemical processes at or near the primitive surface of Mars. And PAHs are among the most common constituents of pollution on Earth; the meteorite could have become contaminated while sitting on the ice. There's no reason to conclude that these PAHs represent the remains of living cells.

  Point two: The carbonate minerals could have formed in many ways other than by circulating water. Carbonates can occur in reactions of rock with carbon dioxide, the most common Martian atmospheric gas. Carbonates commonly grow as alteration products, long after the host rock forms, or directly from melts by igneous processes. Indeed, a number of researchers reanalyzed the minerals and found evidence that they had formed at temperatures well above the boiling point of water.

  Skeptical experts also argued that the minute magnetite crystals prove nothing, since they are common constituents of meteorites that bear no possible signs of life. The chainlike arrays of exceptionally pure magnetite crystals are unusual, to be sure, but most observers feel that magnetite grains are insufficient by themselves to prove the existence of Martian life. Magnetotactic bacteria, furthermore, would have required a moderately strong Martian magnetic field—perhaps stronger than geophysical evidence suggests.

  Finally, the purported fossil microbes are too small—an order of magnitude smaller than any known Earthly bacteria. In fact, they are so small that they could contain no more than a few hundred biomolecules—not nearly enough for a living cell. And there's no reason to characterize them as fossils, since inorganic processes (including sample processing in the lab) are known to produce similar elongated shapes.

  The story became even more confused when scientists began examining other meteorites, Martian and otherwise, in the same meticulous detail afforded the Allan Hills specimen. Surprisingly, all meteorites reveal signs of life—Earth life. Meteorites smash into Earth, where our planet's ubiquitous microbes inevitably contaminate them. Almost every meteorite ever found has lain on the ground for periods ranging from several days to many thousands of years. Once found, they are usually handled, breathed on, and otherwise exposed to more contamination. Unless hermetically sealed almost immediately, any meteorite will be compromised. In a matter of months, microbes migrate deep into a meteorite's interior, exploiting every crack and crevice in a search for the chemical potential energy that is stored in the meteorite's minerals. Given such a messy environment, how could anyone ever be sure about ALH84001?

  One of the most vocal critics of the Martian claim was UCLA paleontologist J. William Schopf. A leading expert on microfossils and an authority on Earth's most ancient life, Schopf was outraged at what he regarded as the NASA team's shoddy analysis and unwarranted conclusions. At the well-publicized August 1996 NASA press conference to discuss the discovery, Schopf was invited to participate as an objective, dissenting voice. “I was like Daniel in the lion's den,” he recalls. Not wanting to publicly denigrate the NASA crowd, he may have pulled his punches in that public forum (“I had tried to be reasonable, even gentle”), but he underscored his criticisms of the NASA work in a scathing addendum to his popular book, Cradle of Life (1999). There he attacked the NASA team with a withering analysis, which he intensified by juxtaposing his critique of ALH84001 with stories of the most egregious paleontological blunders of all time. Of the late famed meteorite, he wrote: “The minerals can't prove it. The PAHs can't either. The ‘fossils' could—but they don't, and there are good reasons to question whether they are in any way related to life.”

  Schopf concluded on a more philosophical note: “There are fine lines between what is known, guessed, and hoped for, and because science is done by real people these lines are sometimes crossed. But science is not guessing.” Little did he suspect that within a few years those righteous proclamations would come back to haunt him.

  EARTH'S OLDEST FOSSILS—THE SCHOPF–BRASIER CONTROVERSY

  The top-down approach to life's origins requires that we ferret out and characterize Earth's most ancient fossil life. Those fragile, fragmentary clues may help us bridge the gulf between geochemistry and biochemistry, and thus deduce key steps in life's emergence.

  Fossil microbial life should be vastly easier to detect in Earth's ancient rocks than in the handful of meteoritic fragments from Mars. After all, we can collect tons of specimens, scrutinize their geological setting, and check any critical measurements in many different laboratories. No matter how remote the rocks or treacherous the journey, it's well worth the effort, for Earth's earliest fossils not only provide a glimpse of the size and shape of ancient life but also reveal the timing of life's opening act.

  Planet Earth formed about 4.5 billion years ago as a giant, molten, red-hot glowing sphere—the result of the accumulation of countless comets, asteroids, and other cosmic debris. For another few hundreds of millions of years, an incessant meteoritic bombardment pulverized every square inch of Earth's surface. What's more, every few million years an epic impact of an object a hundred kilometers or more across punctuated the steady rain of smaller boulders. Such catastrophic events would have repeatedly vaporized any nascent oceans and blasted much of the primitive atmosphere into space. No imaginable life-form could have survived the hellish onslaught of that so-called Hadean eon.

  We don't know exactly when cellular life arose, but the window of opportunity appears to have been surprisingly short. It's almost certain that life could not have persisted before about 4 billion years ago, when the last of the great globe-sterilizing events is estimated to have occurred. It's always possible that life began several times before that, only to be snuffed out by the periodic impact of devastating asteroids. In any case, chemical evidence for life in Earth's oldest known rocks—formations 3.5 to 3.8 billion years old from Greenland, South Africa, and Australia—seem to establish a remarkably ancient lower age limit for life. Such a narrow time window suggests that life's emergence was rapid, at least on a geological timescale.

  Paleontologists devote their lives to scrutinizing fragmentary signs of life in rocks. It's not always a glamorous business, mucking about in inhospitable, remote landscapes, but there's always the possibility for making a big splash. Paleontologists, perhaps more than scientists in any other discipline, can generate gripping headlines. Discoveries of history's biggest shark, most massive dinosaur, or oldest human inspire the public imagination. We live in an age of Guinness-style records; we are obsessed with superlatives. One recent report in USA Today even trumpeted the discovery of the oldest known fossilized penis in a 400-million-year-old crustacean!

  With such a fossil-obsessed press corps, it's little wonder that paleontologist Schopf made the evening news (and Guinness Wo
rld Records) in April 1993 with his announcement in Science of the discovery of Earth's oldest fossils (“Microfossils of the Early Archean Apex Chert: New Evidence of the Antiquity of Life”). Schopf claimed to have identified actual single cells, preserved in the 3.465-billion-year-old Apex Chert from the sun-baked northwestern corner of Western Australia. Even more surprising, these cells occurred in filament-like chains strongly reminiscent of those formed by modern photosynthesizing microbes—cells with the relatively advanced chemical capability to harvest sunlight.

  As in the subsequent ALH84001 incident, the claims were extraordinary and consequently demanded extraordinary proof. In this case, however, the geological community was generally quick to accept Schopf's assertions, because he had established a reputation as one of the world's leading experts in finding and describing ancient single-celled microbes. Schopf and his students had already catalogued dozens of new microbial species from 2-billion-year-old rocks around the world, while establishing rigorous standards for the cautious identification and conservative reporting of new finds. The latest fossils merely pushed back the record for the world's oldest life a few hundred million years.

  A straightforward UCLA protocol had become standard for the maturing field of micropaleontology. Visit Earth's geological formations of the Archean eon (4 billion to 2.5 billion years ago), identify layers of sediment that were deposited in ocean environments, and scour the region for outcrops of distinctive carbon-rich rocks called black chert. Field-workers collect hundreds of pounds of Archean rocks, break off hunks of the most promising specimens, and ship them back to California, where they are sliced into 2 × 3-inch transparent thin sections, a few hundredths of an inch thick.

  The research protocol for finding ancient microbes can be exceptionally tedious. Graduate students are coaxed and coerced into spending thousands of hours examining every part of every slide, micron by eye-straining micron. It turns out that black chert isn't really black at all. Illuminated from beneath and viewed in a powerful microscope, thin sections provide a window on the ancient world. The typical cherty matrix is chockablock full of little black blobs and smudges. Most black chert is seemingly barren of life, but once in a while a thin section reveals a host of tiny spheres, disks, rods, and chains—dead ringers for modern bacteria. Schopf was fortunate that in 1986 one especially sharp-eyed and conscientious student, Bonnie Packer, scrutinized the most promising Australian specimens. Most thin sections yielded nothing of interest, but her discovery of unambiguous microfossils in several ancient units led to a prominent publication and set the stage for the Apex controversy.

  Appearances can be deceiving. Lots of inorganic processes produce round specks and enigmatic squiggles. It's all too tempting to see what you want to see in an ancient rock. That's why Schopf and his colleagues had developed an arsenal of confirmatory tests. For one thing, size matters. Single-celled organisms can't be too small or too big (though some remarkable ancient single-celled organisms are monsters by modern standards). Even more critical, microbial populations tend to cluster tightly around one preferred size, in contrast to the more random sizes of structures produced by nonbiological processes. Consequently, a statistical analysis of size distributions often accompanied Schopf's papers. Uniformity of shape is another key; no fair photographing one or two suggestively contoured black bits while ignoring a multitude of shapeless blobs. Schopf also demanded rigor in the description of local geologic setting and in the proper dating of his samples. As a result, his work on the Apex Chert was initially accepted; he had established a solid reputation for cautious, conservative science.

  But one aspect of Schopf's 1993 study—the claim that some of the microbes were photosynthetic and hence oxygen-producing—remained puzzling. Geochemical evidence from Earth's oldest rocks points to an oxygen-poor atmosphere prior to about 2.2 billion years ago, a time that most researchers identify with the rise of photosynthesis. How could oxygen-producing microbes be present more than a billion years earlier? Nevertheless, within a few years Schopf's claims for the earliest fossils were standard textbook fare; his pictures of Apex fossils had become among the most frequently reproduced of all paleontological images. Schopf himself highlighted the historic findings in Cradle of Life. [Plate 2]

  Controversy erupted in March 2002, after Oxford paleontologist Martin Brasier and a team of seven British and Australian colleagues conducted a careful reexamination of the original type specimens of the Apex Chert fossils, which had been deposited at the Natural History Museum in London. Brasier employed a microscopic technique called image montage, which allowed him to use sharp images of the original thin sections at many different levels within the rock slice to reveal three-dimensional details that were not previously obvious.

  Brasier's microscopic investigation cast the Apex fossils in a new light. Their 3-D structures seemed to differ sharply from those of any known cellular assemblages. In some cases the “filaments” appeared to be more like irregular planes or sheets. In others they branched, a feature never observed with cells. Brasier gave some of the more curious shapes nicknames like “wrong trousers” and “Loch Ness monster.” What's more, the thin sections with the most convincing cell-like objects contained numerous additional black shapes that bore no resemblance at all to cells—forms that Schopf must have seen but failed to detail in his Science paper.

  Further study by Brasier's geological colleagues in Australia pointed to other discrepancies. Schopf had visited the site only briefly and, based on the linear character of the outcrop, reported a classic layered sedimentary sequence with the black chert lying between other layers—a typical ocean-floor scenario. But after detailed field mapping of the site, Australian geologists Martin van Kranendonk and John Lindsay realized that the geological setting of the Apex Chert was much more complex than the simple layered formation Schopf had described. Indeed, the Apex Chert formed at the site of significant hydrothermal activity, where hot volcanic fluids circulated through cracks and fissures. According to their reinterpretation, the black chert formed as a consequence of fluids circulating through this dynamic system as part of a cross-cutting vein. Given this relationship, with the vein of chert cutting across older rocks, the exact age of the Apex Chert was called into question. More damning still, the hydrothermal setting suggested that the chert formed at temperatures far above the permissible limits for life.

  Brasier et al. challenged Schopf's claims in an article titled “Questioning the evidence for Earth's oldest fossils,” published in 2002 in the widely read journal Nature. Their bold conclusion: “We reinterpret the purported microfossil-like structure as secondary artifacts.” The article was a very public attack on Schopf's credibility.

  In an unusual move, the editors of Nature had delayed the Brasier et al. article for more than a year, to allow Schopf time to prepare a rebuttal, “Laser-Raman imagery of Earth's earliest fossils.” The two conflicting articles appeared back-to-back in the March 7, 2002, issue. An accompanying “News and Views” analysis by Nature staffer Henry Gee emphasized the irony of Schopf's predicament.

  Seldom has a scientific debate held such high drama. Schopf had made his reputation in part by staking claim to Earth's oldest life, while cutting no slack for the questionable claims of others. More than any other scientist, he had thrown cold water on the NASA pronouncement of life on Mars. He reveled in reminding the public of past paleontological follies. No wonder then that science journalists were quick to highlight the controversy: “CRADLE OF LIFE OR CAULDRON OF CRUD?” one news headline asked.

  This debate came to a head on April 9, 2002, at the second biennial NASA Astrobiology Science Conference, with Schopf and Brasier squaring off like graying, bespectacled wrestlers. The entertaining spectacle took place deep inside the gargantuan antique dirigible hanger of Moffett Field, 30 miles south of San Francisco, which is home to the NASA Ames Research Center. A sturdy lectern embossed with the NASA logo stood on the stage, to the left of a large projection screen about 12-feet square. B
oth speakers were seated on the stage, before a rapt audience of several hundred scientists.

  Schopf spoke first. A flamboyant presenter even under the calmest of circumstances, Bill Schopf was fighting to preserve his scientific reputation. Barely controlling his anger, his voice booming, he lectured Brasier as if the Englishman were a recalcitrant schoolchild. Step by step, in a talk rich in withering rhetorical questions and exaggerated dramatic pauses, he reviewed the dozen or so necessary and sufficient criteria to establish the authenticity of ancient fossil cells. Step by step, he provided the data to back up his Apex claim, though he did soften his assertion that the microbes were oxygen-producing cyanobacteria.

  After 15 minutes or so, the moderator gestured that Schopf's allotted time was almost up. Like a magician pulling a rabbit out of a hat, Schopf concluded by displaying new analytical data that he claimed would prove his case once and for all. The smudgey black Apex Chert “fossils” are composed principally of carbon, the essential element of life. Carbon concentrations may arise by both biological and nonbiological processes, so carbon in and of itself is not diagnostic of life. However, Schopf claimed, there is a difference: The carbon remains of fossil cells are less perfectly ordered than crystalline carbon deposited as a lifeless mineral. The degree of crystallinity, furthermore, can be revealed by the established technique of Raman spectroscopy. Schopf grandly presented a suite of Raman spectra: Indeed, sharp spiky peaks characteristic of inorganic carbon stood in sharp contrast to the “obviously biological” broad humps in the Raman spectra from the Apex Chert. Schopf concluded by summing up all the evidence he had mustered: “If it fits with all other evidence of life, well follks, most likely it's life.” [Plate 3]

  Brasier gently ascended the stage and began his rebuttal with a dismissive putdown of his rival's presentation: “Well, thank you, Bill, for a truly hydrothermal performance. More heat than light, perhaps.” In soft-spoken Oxford English, the tone in sharp contrast to what had come before, he began to cast doubt on Schopf's case. The most damning evidence were the fossils themselves. With the right lighting, field of view, and level of focus, the Apex features do look like strings of cells. The size is right, the shape more than a little convincing, and there are even regularly spaced dark divisions that look like cell walls. But raise or lower the focus slightly, or shift to another field of view, and doubts arise. What are all those shapeless black blobs next to the “fossil?” How can that supposed straight chain of cells suddenly branch like a “Y”?