H00102--00A, Front mat Genesis

by Charles Baum

  ternal magnets to distinguish “up” from “down,” by sensing the incli-

  nation of Earth’s magnetic field. So sensitive are these organisms to

  their vertical position that magnetotactic bacteria from the Northern

  Hemisphere move in the wrong direction and die when placed in

  Southern Hemisphere soils, where magnetic “up” and “down” are re-

  versed. The NASA scientists claimed that no known inorganic process

  could have produced such an ordered crystalline array.

  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 sug-

  gestive 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 con-

  veyed a sober and reasoned discussion of their findings, and they ac-

  knowledged 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 al-

  ternative explanations for each of these phenomena taken individually,

  when they are considered collectively, particularly in view of their spa-

  tial 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

  36

  GENESIS

  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 aster-

  oids—the raw materials that formed Mars. What’s more, such mol-

  ecules 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 reac-

  tions of rock with carbon dioxide, the most common Martian atmo-

  spheric 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 re-

  quired 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 rea-

  son to characterize them as fossils, since inorganic processes (includ-

  ing sample processing in the lab) are known to produce similar

  elongated shapes.

  The story became even more confused when scientists began ex-

  amining other meteorites, Martian and otherwise, in the same meticu-

  lous 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

  LOOKING FOR LIFE

  37

  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 mi-

  grate deep into a meteorite’s interior, exploiting every crack and crev-

  ice in a search for the chemical potential energy that is stored in the

  meteorite’s minerals. Given such a messy environment, how could any-

  one ever be sure about ALH84001?

  One of the most vocal critics of the Martian claim was UCLA pale-

  ontologist 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 con-

  clusions. 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 inten-

  sified by juxtaposing his critique of ALH84001 with stories of the most

  egregious paleontological blunders of all time. Of the late famed mete-

  orite, 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 ques-

  tion 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 sci-

  ence is done by real people these lines are sometimes crossed. But sci-

  ence 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 biochem-

  istry, and thus deduce key steps in life’s emergence.

  Fossil microbial life should be vastly easier to detect in Earth’s an-

  38

  GENESIS

  cient 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 labora-

  tories. 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 cer-

  tain 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 in-

  spire 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 pe-

  nis in a 400-million-year-old crustacean!

  With such a fossil-obsessed press corps, it’s little wonder that pale-

  LOOKING FOR LIFE

  39

  ontologist Schopf made the evening news (and Guinness World

  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 Aus-

  tralia. 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 extraor-

  dinary 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 doz-

  ens of new microbial species from 2-billion-year-old rocks around the

  world, while establishing rigorous standards for the cautious identifi-

  cation and conservative reporting of new finds. The latest fossils merely

  pushed back the record for the world’s oldest life a few hundred mil-

  lion years.

  A straightforward UCLA protocol had become standard for the

  maturing field of micropaleontology. Visit Earth’s geological forma-

  tions 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 excep-

  tionally tedious. Graduate students are coaxed and coerced into spend-

  ing 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

  40

  GENESIS

  ringers for modern bacteria. Schopf was fortunate that in 1986 one

  especially sharp-eyed and conscientious student, Bonnie Packer, scru-

  tinized the most promising Australian specimens. Most thin sections

  yielded nothing of interest, but her discovery of unambiguous micro-

  fossils 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 pro-

  duce 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 popu-

  lations tend to cluster tightly around one preferred size, in contrast to

  the more random sizes of structures produced by nonbiological pro-

  cesses. 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 ac-

  cepted; he had established a solid reputation for cautious, conserva-

  tive science.

  But one aspect of Schopf ’s 1993 study—the claim that some of the

  microbes were photosynthetic and hence oxygen-producing—re-

  mained 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 photosynthe-

  sis. 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 pale-

  ontological 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 His-

  tory Museum in London. Brasier employed a microscopic technique

  LOOKING FOR LIFE

  41

  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 fea-

  ture 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 ob-

  jects contained numerous additional black shapes that bore no resem-

  blance 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 de-

  scribed. Indeed, the Apex Chert formed at the site of significant hydro-

  thermal 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