Study suggests animal origins may reach back 800 million years

Animals may have originated 800–700 million years ago, according to an Oxford-led study published on October 2 in Science Advances. The analysis challenges a fossil-based limit on their age. Oxford researchers caution that the estimate does not prove animals existed 800 million years ago.

According to Yale University, the oldest recognized animal fossils date to about 574 million years ago. Some recent dating analyses used China's 590-million-year-old Weng'an fossil deposit as a maximum age for animal origins, reasoning that its exceptional preservation would have captured animals if they were present.

Oxford reports that the team examined more than 140 samples from Mongolia's younger Kheseen Biota using scanning electron microscopy. They found finely preserved microscopic organisms, including spiny forms and embryo-like fossils with internal cells, but no definite animals. Animals are known from other locations by this time, undermining that reasoning.

Yale says the researchers instead tested older age limits from deposits in Australia, Norway and Arizona, dating to roughly 850–730 million years ago. They used these to calibrate molecular clocks, which compare genetic differences between living species and fossil reference dates to estimate when ancestors lived. The revised analyses supported animal origins around 800–700 million years ago.

Oxford says this range raises the possibility that animals arose before or during the extreme glaciations associated with Snowball Earth; the Cryogenian Period began about 720 million years ago. Early animals would have been small and soft-bodied, making preservation dependent on their environment and chemical conditions after death. Missing fossils therefore need not mean missing animals.

First author Orin Lole Durbin, now a doctoral student at Virginia Tech, emphasizes that animal body fossils predating the Ediacaran remain elusive. The team calls for searches across deposits with different environments and preservation conditions, combining body fossils, activity traces and chemical biomarkers. Senior author Ross Anderson says the precise timing of animal origins remains uncertain.