Page 206 - Introduction to Paleobiology and The Fossil Record

THE ORIGIN OF LIFE 193

Brasier and colleagues (2002) had argued the month before that Schopf’s “microfossils” were
found in a chert that had not formed in shallow seas, but at high temperature in a hydrothermal
vein. Any microbes in the solidifying rock would have been roasted. So the “microfossils”, said
Brasier, must be inorganic structures. Brasier and his colleagues then examined the original speci-
mens, and found that many had been selectively photographed, so that the full complexity of some
shapes was not seen in Schopf’s published photographs. Many of the “fi laments” were extensions
of more complex blobs and cavities in the chert, and some showed branching and other features
unlikely in a simple prokaryote. Further, the 11 supposed species could not be distinguished, and all
kinds of intermediate shapes were found. Brasier believes the “microfossils” are traces of graphite
in hydrothermal vein chert and volcanic glass. At high temperature the graphite fl owed, forming
black, carbon-rich strings and blobs.
Schopf and colleagues (2002) countered that the carbon traces were formed from living material,
and they applied a new technique, laser Raman spectroscopy, to prove it. They noted that the spectral
bands of the Apex Chert fossils matched signals from known biological materials. But Brasier rebut-
ted this by suggesting that the Raman spectra cannot uniquely identify biological carbon, but simply
match color and grain size between areas of a specimen. Their Raman spectra suggested that the
“microfossils” and the rock matrix consisted of graphite and silica.
Read more about the dispute at http://www.blackwellpublishing.com/paleobiology/. The debate
is renewed in articles by Brasier, Schopf and other commentators in a special issue of the Philosophi-
cal Transactions of the Royal Society in 2006 (Cavalier-Smith et al. 2006).

Figure 8.6 Stromatolites, a Precambrian example from California, USA (magnifi cation ×0.25).
(Courtesy of Maurice Tucker.)