CYANOBACTERIA? Or hydrothermal artifact? NATURE © 2002

The microfossil--if that's what it is--and the others found with it are remarkable because the rock they were found in dates them as a billion years older than any previous evidence for organisms capable of photosynthesis. For life to have evolved to this complexity so early in Earth's history called for a rethinking of how life began on this planet and what conditions it might need to develop elsewhere.

In the intervening years, Schopf and his colleagues have studied what they claim to be microfossils of nearly the same age at two other sites. But the claims, based primarily on the appearance of the microfossils, remain controversial. Now Schopf, graduate student Andrew D. Czaja, and colleagues at the University of Alabama, Birmingham, have used laser-Raman spectroscopic imagery to strengthen the case that these microfossils are exactly what the researchers have always claimed they were [Nature, 416, 73 (2002)].

But critics of the claim, led by geochemist Martin D. Brasier of the University of Oxford, are not convinced. Using their own laser-Raman spectroscopic imaging as well as a computer-aided optical imaging technique called automontaging, they posit that the structures are filaments of carbon that could have formed by Fischer-Tropsch chemistry in an ancient hydrothermal system [Nature, 416, 76 (2002)].

Laser-Raman spectroscopic imaging, both groups agree, shows convincingly that the structures contain particulate carbon with an isotopic ratio that's consistent with a biological origin for the material. Brasier claims that the carbon is in the form of graphite; Schopf says it's kerogen, a polycyclic aromatic hydrocarbon (PAH) generally taken to be biologic in origin when it's found on Earth.

But, Brasier points out, both PAHs and carbon with this isotopic ratio are also found in some meteorites, where their origin is assumed to be nonbiological. "Structures cannot be called biogenic until all possibilities of their abiogenic origin have been exhausted," Brasier says. "We were astonished to find that these so-called microfossils are bound up in elemental iron and nickel and tin. That is exactly what you would predict if this is organic material synthesized within a hydrothermal environment and recrystallized later on."

Schopf disagrees with this analysis. "Particulate organic matter produced by Fischer-Tropsch or any other nonbiologic process has never, ever, been reported in the geological record," he says. "We have now done analyses of 25 fossil-bearing rocks ranging in age from 400 million years to 3.5 billion years, and these oldest samples fit right in. It's a really nice study showing the geochemical changes that take place in organic matter in sediments."

Other researchers in the field are taking a more middle ground. Since Fischer-Tropsch chemistry has not been clearly seen in nature before, a biological origin for the carbon in the specimens seems more likely, argues geochemist George D. Cody of the Carnegie Institution of Washington. Yet, he adds, the claim that these specimens represent cyanobacteria needs to be more firmly established.

"At the end of the day," Cody says, "both studies convince me that it is a difficult business to study the vestiges of ancient life in some of the most ancient rocks."

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