The first animals may have lived 800 to 700 million years ago. That is about 200 million years earlier than many recent estimates, which leaned on the oldest fossil evidence, according to a study published on October 2 in Science Advances.
The question is old: genes suggest animals are ancient, while rocks show them late. Scientists estimate when groups split apart with a molecular clock, a method that counts genetic differences between living species and turns them into time. The clock needs fossils as reference points, and those reference points decide the answer.
A fossil site with no animals
The team, led by Orin Lole Durbin, now at Virginia Tech, with Ross Anderson of the University of Oxford, looked again at those reference points. A key one is the Weng'an fossil site in China, about 590 million years old, which has been used to cap how old animals could be.
The researchers examined more than 140 microfossil samples from the Kheseen Biota in Mongolia with a scanning electron microscope. The site is more than 40 million years younger than Weng'an and just as well preserved. It has no recognizable animals either, even though animals certainly lived by then.
"The Kheseen microfossils are just as well preserved, yet animals continue to be absent," Anderson says. So a missing animal in a good fossil site does not prove animals did not exist yet.
Still no bodies
Without that cap, the clock points much further back. The authors are careful about what it means.
"Pre-Ediacaran animal body fossils still elude us, and this analysis does not prove that animals existed 800 million years ago," Durbin says. The earliest animal body is still missing.
