Odd-shaped mounds of dirt in Australia turn out to be fossils of the oldest life on Earth, created by billions of microbes more than 3 billion years ago, scientists say in a new report.

And these mounds are exactly the type of life astrobiologists are looking for on Mars and elsewhere.

A study published Thursday in the journal Nature gives the strongest evidence yet that the mounds dotting a large swath of western Australia are Earth's oldest fossils.

The theory is that these are not merely dirt piles that formed randomly into odd shapes, but that ancient microbes burrowed in and built them.

"This is the pointy end of the fossil record; this is the first really compelling record," said study lead author Abigail Allwood, a researcher at the Australian Centre for Astrobiology. "It's an ancestor of life. If you think that all life arose on this one planet, perhaps this is where it started."

The mounds come in different shapes — like egg cartons, swirls of frosting on cupcakes or waves on the ocean. They are called stromatolites and have been studied for a long time, but the big question has been if they were once teeming with life.

Allwood's research, which included examining thousands of the mounds and grouping them into seven subtypes, is the most comprehensive and compelling yet to say the answer is yes, according to a top expert not on her team.

"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."

Allwood said her study made the case for life solidly by looking at how the stromatolites fit with the rock formations around them, with each other, and what would have been happening on Earth at that time.

One of the clinchers was putting them in 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 Australian 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.

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."