This fossil is one of the world’s earliest animals, according to fat molecules preserved for a half-billion years

For more than 70 years, scientists have puzzled over the bewildering shapes of half-a-billion-year-old fossils that don’t look like any other organisms that have ever lived on Earth. Paleontologists haven’t even been able to tell whether many of these oddly shaped fossils from ancient oceans represent plants, animals, or some other life form. Now, traces of cholesterol—a signature of animal life—from a set of incredibly well-preserved fossils confirm that a creature called Dickinsonia, which looks a bit like a quilted bath mat, was in fact a strange animal.

These chemical traces “are giving us a completely different way of understanding what is happening” in very ancient ecosystems, says paleontologist Douglas Erwin of the Smithsonian Institution’s National Museum of Natural History in Washington, D.C.

The life forms that lived on Earth half a billion years ago left behind some of the oddest fossils known. Called Ediacarans—named for Australia’s Ediacara Hills, where some of the first ones were found—they lived in the oceans between 570 million and 541 million years ago, just before the Cambrian explosion, when the first recognizable animals emerged. Up to nearly a meter long, some of the 200 described Ediacaran species have fractal-shaped fronds. Others, like Dickinsonia, look like they had fluid-filled modules that gave them a “quilted” appearance. Theories about what they were abound: giant protists? Lichens? Algae? Some sort of sponge? Or some other form of life that has since disappeared?

There are some clues. Evidence suggests some Ediacara moved, and studies of the way they seemed to grow have suggested at least some of the creatures were animals, including Dickinsonia. Others were probably colonies of bacteria or algae.

Ilya Bobrovskiy, a geologist who now works at the Australian National University (ANU) in Canberra, wondered whether he might be able to get clues from some exceptional fossils that still preserve a film of what looks like organic material. These fossils come from a cliff on the shore of the White Sea in northwestern Russia, where for 550 million years, the rocks have escaped the heat and pressure that can obliterate molecular traces. “They are some of the least cooked rocks of this age that anyone has found,” Erwin says.

Bobrovskiy contacted Jochen Brocks, an expert in ancient biomolecules at ANU, to ask whether he thought the film might still contain molecules that could reveal clues to the organism. “Jochen said I was completely insane,” Bobrovskiy says. (Brocks’s version of the story: “I said, ‘Right. That’s the most stupid idea I’ve ever heard.’ But I told him he should find out for himself.”)

Bobrovskiy moved from Russia to Australia to join Brocks’s lab. There, he first tested his idea on a collection of small round Ediacaran fossils called Beltanelliformis. The researchers removed the film from the rock, dissolved it, and used gas chromatography and mass spectrometry to look for preserved organic molecules. They found high levels of hopanes, a molecule that suggested the Beltanelliformis were colonies of cyanobacteria, the researchers reported earlier this year.

Buoyed by that result, the researchers had the nerve to try the technique with the much larger Dickinsonia fossils. (“They would have brought $30,000 on eBay,” Brocks says. “But we sliced them up and dissolved them.”) The results, reported today in Science , were striking. In the Dickinsonia fossil, 93% of the organic molecules had 27 carbon molecules; that makes them members of a family called cholesteroids, which includes cholesterol and is a signature of animal cells. Samples from immediately above and below the Dickinsonia fossil had a different mix of steroids: Only 11% were cholesteroids and more than 70% were stigmasteroids, molecules with 29 carbon atoms, which are a signature of green algae.

“It’s a very unusual style of organic preservation,” says Gordon Love, a geochemist at the University of California (UC), Riverside. “We don’t usually expect to find these organic films, so there are a few quirky features that require more evaluation.” Still, he says, the conclusion that Dickinsonia produced cholesterol—and was therefore an animal—“is the most parsimonious explanation at this stage. I would say it’s plausible.”

“If this were the only evidence we had, it wouldn’t be enough,” says Mary Droser, a paleontologist and expert on Ediacarans at UC Riverside. “But along with the other evidence, it’s great.” She says it’s especially satisfying that the biochemical evidence came from Dickinsonia. “I won’t say it’s the [Tyrannosaurus] rex of the Ediacarans—it’s not a predator. But it has that sort of standing,” she says, as an iconic representative of its ecosystem. “It is a special one, and it’s wonderful to have evidence from the geochemical world that it was an animal.”