Page 205 - Introduction to Paleobiology and The Fossil Record

192 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

their intake of sunlight, but from time to time ing microbes are not preserved, but the layered
the mat is overwhelmed by sediment. The structure remains. Many early examples have
microbes migrate towards the light, and recol- proved controversial, but the oldest that are
onize the top of the sediment layer, which may generally accepted come from Australia, and
again be swamped by gentle seabed currents. are dated as 3.43 Ga (see p. 290).
Over time, extensive layered structures may Perhaps the oldest currently accepted fossils
build up. In freshwaters, and sometimes in the other than stromatolites date from 3.2 Ga.
sea, stromatolites build up by precipitation of They were found in Western Australia by
calcite. In most fossil examples, the construct- Birger Rasmussen, and reported in 2000, from

Box 8.1 The Apex Chert: oldest life or hot air?

There was a sensation when Bill Schopf announced the world’s oldest fossils in 1987 (Schopf &
Packer 1987). He later reported a diverse assemblage of 11 species of bacteria and cyanobacteria
from the Apex Chert of the Warrawoona Group in Western Australia, dated as 3465 Ma (Schopf

1993). All specimens are filament-like microbes, ranging in length from 10 to 90 μm; some are cir-

cular single cells, while most are filaments consisting of several compartments (Fig. 8.5). These were
widely accepted as genuine fossils, and they featured in all the textbooks and web sites as real
examples of the earliest cyanobacteria and bacteria.
But their validity was challenged in April 2002. At the second Astrobiology Science Conference
held at NASA’s Ames Research Center in Moffett Field, California, there was a bombshell. As
reported in Nature:

It was the academic equivalent of a heavyweight prizefight. In the red corner, defending his

title as discoverer of the Earth’s oldest fossils, was Bill Schopf of the University of California,
Los Angeles (UCLA). In the blue corner, Martin Brasier of the University of Oxford, UK, who
contends that Schopf’s “microfossils” are merely carbonaceous blobs, probably formed by the
action of scalding water on minerals in the surrounding sediments.

0 10 μm 20

Figure 8.5 Postulated prokaryotes from the Apex Chert of Western Australia (c. 3465 Ma)
showing filament-like microbes preserved as carbonaceous traces in thin sections. All are examples

of the prokaryote cyanobacterium-like Primaevifi lum, which measures 2–5 μm wide. (Courtesy of
Bill Schopf.)