The tiny organisms were encased in chert, a fine-grained sedimentary rock. On the basis of morphology (shape), Schopf sorted the bacteria into 11 taxa, or distinct groupings, and said 7 were probably early relatives of cyanobacteria, primitive photosynthetic organisms.

Cyanobacteria are credited with the first large-scale production of atmospheric oxygen. By converting a reducing (oxygen-poor) atmosphere to an oxidizing (oxygen-rich) one, cyanobacteria permitted the evolution of more advanced, oxygen-breathing organisms.

Yet the first geochemical evidence of significant atmospheric oxygen – evidence preserved in the chemical makeup of ancient rocks – came from about 1 billion years later.

Schopf’s identification, which gained acceptance as the first evidence of oxygen-producing organisms, is now under bruising attack. Critics charge not only that the structures Schopf observed in the rocks are not fossilized cyanobacteria, but also that they are not biological fossils at all.

Martin Brasier, professor of paleobiology at Oxford University, along with an international team, raised numerous counter-arguments in the March 7, 2002, edition of Nature. Brasier says Schopf misunderstood the geology of the supposed microfossils, which were preserved not in marine sediments, but rather in a hydrothermal vent – or even in volcanic glass.

"The context for the Apex chert 'microfossils' can be reconstrued as the subsurface feeder dyke to a submarine (possibly quite deep) hydrothermal vent, closely associated with volcanicity." (In other words, they formed in association with hot fluids near a volcanic structure.) The hot, low-oxygen conditions may have been "excellent for the prebiotic synthesis" of organic compounds, Brasier adds, but at several tens of meters below the vent, were "not a happy habitat for oxygen-producing cyanobacteria."

In an email, Brasier also charged that Schopf overstated or misinterpreted the regularity in the shape of his specimens. "Eleven fossil taxa (the greatest diversity in the fossil record until the Gunflint chert, some 1600 million years younger) were … based upon structures that are much more chaotic and inconsistent than shown and described."

This business of shape is important because Schopf used physical appearance as the basis of his original identification. Brasier says his examination of the specimens revealed structures "indistinguishable from graphitic mineral growths that occur alongside [the purported fossils] - they frequently branch chaotically in the same way and intergrade continuously with the shapes of inanimate matter."

Appearance is a poor guide to biogenic origin, Brasier concludes. "This provides a clear warning to all astrobiologists that complex, microfossil-like structures can easily arise from simple, abiogenic causes."

Only by carefully comparing non-biological and biological structures, he says, will it be possible to distinguish structures made by biology.

Schopf is having none of this. For starters, he calls the argument over cyanobacteria a "red herring. I have referred to these specimens as 'cyanobacterium-like,' 'possible cyanobacteria,' and even 'probable cyanobacteria,' but I have never, ever claimed that they were cyanobacteria per se. Nobody goes to look at the literature. I defined these taxa as being of uncertain systematic position. I said they were prokaryotes [microorganisms without cell nuclei], and still think they are, but I didn't know for sure then, and I don't know now, precisely what kinds of bacteria they are."

As for Brasier’s contention that fossils could not be found in a hydrothermal or volcanic structure, Schopf says that, while the conditions of deposition remain contentious, "It’s perfectly fine for there to be organisms in hot springs. Hot springs today are teeming with life; think of Yellowstone. If there was life around [3.5 billion years ago], it would be astounding that it was not present in such a hot-spring environment."

Schopf says his scientific approach is similar to that of Georges Cuvier, a founder of paleontology who first applied the Linnaean classification system to fossils. "Cuvier said that the way you solve the problem [of determining whether or not a specimen is biogenic] is that you identify a suite of traits that are unique to life, as a suite, and that not exhibited by inanimate matter, but are exhibited by living organism and the fossils."

The Apex chert specimens satisfy critical criteria for fossils, Schopf continues. "Every test that can be applied has been applied, and the Apex fossils meet them all, a suite of traits that taken together are simply unknown in nonbiologic materials."

Here's his argument:

• Morphology: 200 specimens from the Apex rocks and numerous specimens from other rocks of similar age show the characteristic shapes of bacteria.
• Consistency: The fossils make sense in the context of other information about the epoch, and are not, for example, unduly advanced or retarded compared to other evidence from the same period. "There are hundreds of Precambrian fossil-bearing chert units like the Apex, and they have the same sorts of fossils," he says.
• Isotopic analysis: 10 analyses of the Apex charts, and 125 analyses on other fossil units of about the same age "all show the same distinctive biological [carbon isotope] signal."
• Composition: Raman imagery of 15 specimens from the Apex and other cherts of the same age, Schopf says, show the remains of organic matter. (Raman imagery is an analytical technique that can map the distribution of a particular chemical throughout a sample. It enables one to compare the location of specific chemical signals to visible shapes within the sample.

But not everyone is convinced by Schopf's data. At a recent Geological Society of America conference, Jill Pasteris, a professor of earth and planetary sciences at Washington University who has used Raman spectroscopy for 20 years, presented a poster showing that Schopf's conclusion about kerogen was "erroneous."

Kerogen is a grab-bag description of the remains of organic matter as it gradually decomposes, says Pasteris. She and Brigitte Wopenka, also of Washington University, presented Raman spectra from non-biogenic carbon material at the GSA "that looked just like the spectra that Schopf et al produced in Nature. In other words, the spectra in Nature are the spectra that one gets from disordered carbonaceous material, whether or not it's biogenic in origin."

Can Raman spectroscopy "really distinguish this kerogen from some other yucky carbon material? The answer is no," Pasteris told Astrobiology News.

In other words, says Pasteris, the fact that Schopf was able to match up the spectral signatures of certain carbon compounds with visible shapes within his samples doesn't prove their biological origins, because the spectral signatures themselves were inconclusive.

Fossil claims, Brasier writes in his email, "are better tested by falsification rather than by justification. To that end, our research group urges that future studies (older than, say, 3.0 billion years) should explore the following null hypothesis: that very ancient/alien microfossil-like structures … should not be accepted as of biological origin until all possibilities of their non-biologic origin have been tested and can be falsified."

The debate does highlight a need for clearer standards of evidence for matters paleontological and astrobiological. Ironically, despite more data, improved techniques, and increased focus on the field, scientists are finding that one of the most difficult questions to answer is also one of the most basic: "How do we really know?"

By David Tenenbaum, Astrobiology Magazine

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