"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 that 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.
Some look like frosting swirls on cupcakes; others look like the inside of an egg carton. Allwood even nicknamed one 6½-foot mound "crocodile back" because of its appearance.
Stromatolites have been studied for a long time, but the big question has been: Were they once teeming with life? Recently, more scientists have been leaning toward answering yes.
Allwood's research included examining thousands of the rocky mounds and grouping them into seven subtypes. It is the most comprehensive and compelling evidence that these are fossils of life, not funny-shaped rocks, 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 by looking at how the stromatolites fit with the rock formations around them, with each other, and with what would have been happening on Earth at that time. 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.