Geologists report finding Earth’s oldest known fossils

NEW YORK – A team of Australian and British geologists has discovered fossilized, 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, a reign of terror in which 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 sterilized 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 yesterday’s 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 was a beach 3.4 billion years ago, on one 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 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, the 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.

Microfossils – the cell-like structures found in ancient rocks – have become a highly contentious field, 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 honor of having found the most ancient microfossil has been 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 today’s 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.’’

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