Fossil imprints that resemble the rippled underside of a mushroom’s cap are remnants of the oldest-known animals in Earth’s history. The finding, published in Science on 20 September1, is based on a chemical analysis of fatty molecules preserved in the fossils. It could alter the current story of how animals and other complex life arose.
Researchers first discovered the pancake-shaped creatures — a group known as Dickinsonia — in the late 1940s. The species were among the most common residents of the world’s oceans 558 million years ago, during the Ediacaran period. Whereas most living things during that time ranged in size from microscopic to a few millimetres long, some Dickinsonia grew up to 1.4 metres in length.
The creatures’ large size has puzzled scientists because Dickinsonia lived tens of millions of years before the Cambrian explosion, the period 541 million years ago when living things became bigger and most major animal groups emerged. Scientists have since debated whether Dickinsonia were primitive animals, giant single-celled organisms called protists, bacterial colonies or something else entirely.
The latest study attempts to end that debate by analysing chemical biomarkers preserved in a unique set of Dickinsonia fossils from Russia, rather than by examining the ancient species’ body characteristics.
A team led by Jochen Brocks, a palaeobiogeochemist at the Australian National University in Canberra, examined ring-like fat molecules called sterols that infiltrate the membrane surrounding a cell to keep it flexible and fluid. Plants, animals, fungi and bacteria all contain sterols, but the type of sterol that predominates in each group differs. Animals mainly make cholesterol, and the fungi that form colourful, crusty lichens found on boulders have only ergosterol. Under the right conditions, these chemicals can persist for millions of years, and so help to determine a fossilized organism’s evolutionary relationships.
Fossils that contain these preserved biomarkers are rare, but strewn around the shores of the White Sea in northwestern Russia lie Ediacaran fossils — including Dickinsonia — embedded in a fossilized mat of algae, with their organic matter and fats perfectly preserved. “They are, in principle, mummified dickinsonians,” Brocks says. “It’s just incredibly lucky.”
But the team’s analysis revealed dramatic differences in the composition of the biomarker samples. Whereas the surrounding rocks and algal mats contained only about 10% cholesterol and 75% of another sterol that is common in green algae, the Dickinsonia fossils contained 93% cholesterol — suggesting that they were ancient animals living 17 million years before the Cambrian explosion.
The technique provides an entirely different way of determining Dickinsonia’s place on the evolutionary tree, says Guy Narbonne, a palaeobiologist at Queen’s University in Kingston, Canada. “I think it’s quite imaginative.” The findings from the chemical analysis reinforce other evidence that Dickinsonia were primitive animals, he says. This includes fossil ‘footprints’ that show the organisms moved from place to place for food, and growth patterns that match those of most animals today.
The latest findings also suggest that the transition between the Cambrian and the Precambrian, which includes the Ediacaran, was just another extinction event in which one animal community replaced another, Brocks says. “But now the jury’s out on all the other weirdos.”
Analyses that have compared the DNA of living creatures today to trace back their evolutionary trees suggest that animals originated more than 100 million years before the Cambrian — well before even Dickinsonia2. But finding the fossils of these creatures, and then proving they are animals, remains challenging.
Rangeomorphs, strange Ediacaran frond-like creatures with tubes branching in a fractal pattern, for example, remain a mysterious group whose relation to any living organisms is uncertain. “This would be for us the next big challenge,” Brocks says. “Trying to get hold of those strange creatures and find out what they were.”