WASHINGTON (AP) – The best evidence yet for the oldest life on Earth is found in odd-shaped, rock-like mounds in Australia that are actually fossils created by microbes 3.4 billion years ago, researchers report.

“It’s an ancestor of life. If you think that all life arose on this one planet, perhaps this is where it started,” said Abigail Allwood, a researcher at the Australian Centre for Astrobiology and lead author of the new study. It appears Thursday in the journal Nature.

The strange geologic structures – which range from smaller than a fingernail to taller than a man – are exactly the type of early life astrobiologists are looking for on Mars and elsewhere.

They are known as stromatolites. They’re produced layer by layer when dirt sediments mix with carbon dioxide expelled from bacteria, water, and minerals – all trapped in the microbes’ sticky mucilage.

The theory is that these ancient mounds dotting a large swath of western Australia are not merely dirt piles that formed randomly into odd shapes, but that microbes built them a few billion years ago.

“It is the best bet for the best evidence of the oldest life on Earth,” said Bruce Runnegar, director of the NASA Astrobiology Institute in Moffett Field, Calif. “These are too complicated to be attributed to non-biological processes – but we don’t know that for a fact.”

One of the clinchers was categorizing them into seven repeating subtypes, which indicates they weren’t random.

“It’s just the sheer abundance of material and to be able to put it all in context,” Allwood said.

Runnegar who has examined the mounds in western Australia several times said the first time he saw them – some of which jut out from hills at eye-level – he experienced an otherworldly feeling.

In a similar situation 10 years ago, scientists at NASA claimed they found evidence of fossilized microbial life in a Martian meteorite.

Those claims have been sharply disputed and are not generally accepted.

One of the chief skeptics of the Martian meteorite claims, Ralph Harvey, a geology professor at Case Western Reserve University in Cleveland, said he is far more inclined to believe that the Australian mounds were once alive.

The key difference is that on Mars, scientists were looking for evidence of life on “a potentially dead planet” and the requirement for proof is extraordinary, Harvey said. Less evidence, he said is needed for Allwood’s claims because “we already know that life has been on Earth for a very, very long time; all we’re trying to do is push it further back.”

On the net: http://www.nature.com/nature

AP-ES-06-07-06 1554EDT

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