Washington ? 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 today 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.

This photo shows a fossil, outlined with white dashes, that researchers call crocodile

“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 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.