Dustup Over Oldest Fossils
A set of fossils described in many textbooks as the oldest ever found may be nothing more than intriguingly shaped, but purely lifeless minerals, according to a new report.
In 1993, paleontologist William Schopf of the University of California, Los Angeles (UCLA), claimed that the microscopic squiggles in a 3.5-billion-year-old Australian chert were fossilized bacteria (Science, 30 April 1993, p. 640). Now, in the 6 March issue of Nature, a team led by Martin Brasier, a micropaleontologist at the University of Oxford, United Kingdom, argues that the squiggles were formed by ancient hot-spring chemistry. If true, his analysis would call into question the fossil record of life's first billion years. It would also raise doubts about the judgment of Schopf, the man chosen by NASA to set the standard for distinguishing signs of life from nonlife at the press conference unveiling martian meteorite ALH84001 (Science, 16 August 1996 p. 864).
Examining Schopf's sections of chert, Brasier saw the same segmented, wormlike threads of dark organic matter pictured in the paper. But he saw a great deal more when he focused the microscope above or below these shapes. A long, stringy "microfossil"--seen with sharp terminations at either end in Schopf's image--extended downward from one end and ballooned to many times its original width. Some strands seemed to branch--something chains of bacterial cells generally don't do. Other structures ranged from vaguely suggestive of life to inscrutable jumbles, Brasier reports. "Parts that would look like a bacterium [at one focal depth] took on weird shapes" at other depths, he says. The team suspects that the shapes are organic matter shaped by hot-spring minerals as they grew and transformed on cooling.
But Schopf says Brasier's interpretation "is just a mistake; they haven't the experience looking at Precambrian microfossils, or such a depth of focus confuses them." The very unbacterial branching of chains of bacterial cells, for example, is actually folding of chains, he says.
Schopf has some support in the Precambrian community, but other Precambrian paleontologists side with Brasier. "The structures illustrated by Schopf as microfossils are part of a greater galaxy of structures that clearly are not biological," says Andrew Knoll of Harvard University. Whether the textbooks get rewritten will depend on an analysis of material from new sites, says Malcolm Walter of Macquarie University in Sydney. "I doubt if it will be resolved by more clever work on these samples," he says. "It gets resolved by more work on more rocks."