David Wacey

Their assertion, if sustained, confirms the view that life evolved on earth surprisingly soon after the Late Heavy Bombardment, a reign of destruction in which waves of asteroids slammed into the primitive planet, 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.

The claim is also a new volley in a long-running conflict over who has found the oldest fossil.

The new microfossils are described in Sunday’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. The fossils 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 had heated the oceans to the temperature of a hot bath.

It was in these conditions, the geologists believe, that organisms resembling today’s bacteria lived in the crevices between the pebbles on the beach. 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 — many have turned out to be artifacts formed by nonbiological processes. In this case, the geologists have gathered considerable circumstantial evidence that the structures they see are biological. With an advanced new technique, they have analyzed the composition of very small spots within the cell-like structures. “We can see carbon, sulfur, nitrogen and phosphorus, all within the cell walls,” Dr. Brasier said.

Crystals of fool’s gold, an iron-sulfur mineral, lie next to the microfossils and indicate that the organisms, in the absence of oxygen, fed off sulfur compounds, Dr. Brasier and his colleagues say.

Microfossils — the cell-like structures found in ancient rocks — have become a highly contentious field, both because of the pitfalls in proving that they 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, Dr. Schopf reported his discovery of fossils 3.465 billion years old in the Apex chert of the Warrawoona Group in Western Australia, about 20 miles from where the new fossils have been found. Those would be some 65 million years older than the new find, but Dr. Schopf’s claim was thrown in doubt in 2002 when Dr. Brasier attacked his finding, saying the fossils were not biological but just mineral artifacts.

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

The Nature Geoscience article published on Sunday does not claim discovery of the earth’s oldest microfossils. That assertion was made in a press release issued by the University of Oxford, where Dr. Brasier is a professor in the department of earth sciences.

Dr. Brasier said the article submitted to Nature Geoscience had made such a claim, but the reviewers questioned the advisability of doing so, and the senior author, Dr. Wacey, “decided to acquiesce on this particular point.”

Dr. Schopf did not respond to an e-mail 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.

Dr. Buick said there was no consensus on Dr. 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 Dr. Wacey and Dr. Brasier has made a “pretty good case,” Dr. 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,” Dr. Buick said.

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,” Dr. Buick said.

Andrew H. Knoll, an earth scientist at Harvard, said in a brief e-mail from a Moscow airport that the researchers had not proved their point that the fossils, when alive, fed on sulfur compounds. But he did not take sides on the dispute between Dr. Brasier and Dr. Schopf.

Dr. Buick said: “You’ve got to realize how divisive this microfossil war has been over the last decade. Most people just want it to be over. If claim and counterclaim go back and forth for a decade, it sounds like we don’t know what we’re doing.”