Oldest known fossils found?

A team of Australian and British geologists have discovered fossilised, single-celled organisms that are 3.4 billion years old, which they say are the oldest known fossils on earth.

The claim, if sustained, confirms the view that life evolved on earth surprisingly soon after the Late Heavy Bombardment, when waves of asteroids slammed into the primitive earth, heating the surface to molten rock and boiling the oceans into an incandescent mist. The bombardment, which ended around 3.85 billion years ago, would have sterilised the earth’s surface of any incipient life. It is also a new volley in a long-running conflict over who has found the oldest fossil.

The new microfossils are described in a recent issue of Nature Geoscience by a team led by David Wacey of the University of Western Australia and Martin D Brasier of the University of Oxford in England. They were found in sandstone at the base of the Strelley Pool rock formation in Western Australia. The sandstone, 3.4 billion years ago, was a beach on one of the few islands that had started to appear above the ocean’s surface.

Conditions were very different from those of today. The moon orbited far closer to earth, raising huge tides. The atmosphere was full of methane, since plants had not yet evolved to provide oxygen, and greenhouse warming from the methane heated the oceans to the temperature of a hot bath.

It was in these conditions that organisms resembling today’s bacteria lived in the crevices between the pebbles on the beach, geologists believe. Examining thin slices of rock under the microscope, they have found structures that look like living cells, some in clusters that seem to show cell division. Cell-like structures in ancient rocks can be deceiving as “many have turned out to be artifacts formed by non-biological processes.

In this case, the geologists have gathered considerable circumstantial evidence that the structures they see are biological. They have analysed the composition of very small spots within the cell-like structures. “We can see carbon, sulfur, nitrogen and phosphorus, all within the cell walls,” Brasier said. Crystals of fool’s gold, an iron-sulfur mineral, lie next to the microfossils and indicate the organisms, in the absence of oxygen, fed off sulfur compounds, Brasier and colleagues say.

Contentious subject
Microfossils – the cell-like structures found in ancient rocks – have become contentious, both because of the pitfalls in proving the structures are truly biological, and because the scientific glory of having found the oldest known fossil has led to pitched battles between rival claimants.

The honour of having found the most ancient microfossil has long been held by J W Schopf, a paleobiologist at the University of California, Los Angeles. In 1993, Schopf reported his discovery of fossils 3.465 billion years old in the Apex chert of the Warrawoona Group in Western Australia, some 20 miles from where the new fossils have been found. Those would be some 65 million years older than the new find, but Schopf’s claim was suddenly thrown in doubt in 2002 when Brasier attacked his finding, saying the fossils were not biological but just mineral artifacts.

With the latest finding, Brasier has dropped his second shoe, claiming discovery of microfossils that are or may be the oldest known, if and when Schopf’s are knocked out of the running.

The Nature Geoscience article does not claim discovery of the earth’s oldest microfossils. That claim is made in a press release issued by Brasier’s college, the University of Oxford. Brasier said the article submitted to Nature Geoscience had made this claim but the reviewers had questioned the advisability of doing so and the senior author, Wacey, “decided to acquiesce on this particular point.”

Schopf did not respond to an email seeking his comments. “Bill Schopf still very strongly defends his original claim, and is working to validate it,” said Roger Buick, an earth scientist at the University of Washington in Seattle. Buick said there was no consensus on Schopf’s microfossils, but that “the majority opinion is that they are probably not biological and probably not as old as claimed.”

The team led by Wacey and Brasier has made a “pretty good case,” Buick said, because the many different analytic techniques they have used “lend credence to the argument in a way that many other previously reported discoveries of particularly ancient microfossils have not.” Does that mean the new microfossils are the oldest known? “If these are valid, and if we discount the Schopf microfossils, these would be the oldest known, though not by much,” Buick said.

Traces of life in ancient rocks
Rocks older than 3.5 billion years have been so thoroughly cooked as to destroy all cellular structures, but chemical traces of life can still be detected.

Chemicals indicative of life have been reported in rocks 3.5 billion years old in the Dresser Formation of Western Australia and, with less certainty, in rocks 3.8 billion years old in Greenland. “This struggle to be the owner of the world’s oldest microfossils is really not the crux of the battle for understanding the early development of life anymore,” Buick said.