Dickinsonia lived in the oceans more than 540 million years ago.

Ilya Bobrovskiy/Australian National University

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

By Gretchen VogelSep. 20, 2018 , 2:00 PM

Por mais de 70 anos, os cientistas têm intrigado as formas desconcertantes de fósseis de meio bilhão de anos que não se parecem com nenhum outro organismo que já viveu na Terra. Os paleontólogos não foram capazes de dizer se muitos desses fósseis de forma estranha de oceanos antigos representam plantas, animais ou alguma outra forma de vida. Agora, vestígios de colesterol - uma assinatura da vida animal - de um conjunto de fósseis incrivelmente bem preservados confirmam que uma criatura chamada Dickinsonia, que se parece um pouco com um tapete de banho acolchoado, era na verdade um animal estranho.

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.

As formas de vida que viveram na Terra há meio bilhão de anos deixaram para trás alguns dos mais estranhos fósseis conhecidos. Chamados de Ediacarans - batizados com o nome de Ediacara Hills da Austrália, onde alguns dos primeiros foram encontrados - eles viveram nos oceanos entre 570 milhões e 541 milhões de anos atrás, pouco antes da explosão cambriana, quando surgiram os primeiros animais reconhecíveis. Até quase um metro de comprimento, algumas das 200 espécies descritas de Ediacaran têm frondes em forma de fractal. Outros, como Dickinsonia, parecem ter módulos cheios de fluido que lhes davam uma aparência “acolchoada”. Teorias sobre o que eles eram abundantes: protistas gigantes? Liquens? Algas? Algum tipo de esponja? Ou alguma outra forma de vida que desapareceu desde então?

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.

A film of organic matter from an unusual ancient fossil

Ilya Bobrovskiy/Australian National University

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.”

Posted in:

• Paleontology

doi:10.1126/science.aav4877