Animals May Have Emerged 200 Million Years Before Earliest Fossils, Study Suggests

A new modeling study indicates that the first animals may have appeared on Earth hundreds of millions of years before their fossil record suggests. While the earliest undisputed animal fossils date to approximately 574 million years ago during the Ediacaran period, researchers argue that the evolutionary origins could extend back between 800 million and 700 million years ago.

Published on October 2 in the journal Science Advances, the research challenges the reliance on sparse fossil evidence. The team posits that the absence of animal fossils in certain well-preserved ancient rock layers does not necessarily mean animals were absent from that era. Instead, gaps in the fossil record may conceal a significant chapter of early animal evolution, potentially placing their emergence deep within the Neoproterozoic era, possibly before or during some of Earth’s most severe ice ages.

Previous studies often pointed to the Weng’an Biota in China, dating back roughly 590 million years, as a benchmark. This deposit contains uniquely preserved microscopic organisms but lacks definitive animal fossils, leading some scientists to view it as an upper limit for animal evolution. However, the new study questions whether this logic holds when examining other fossil sites from periods known to contain animal life.

To test this hypothesis, lead researcher Orin Lole Durbin and his colleagues analyzed more than 140 samples from the Kheseen Biota in Mongolia. This deposit is over 40 million years younger than the Weng’an site. Using electron scanning microscopy, the team observed microscopic fossils with extraordinary detail, including spherical organisms and embryo-like structures with internal cells. Despite the high quality of preservation, no microfossils could be identified as animals.

“The Kheseen Biota breaks the argument that the exceptional microfossils of Weng’an mean we would have seen animal fossils in the assemblage had they existed at the time,” said Ross Anderson, an associate professor of natural history at the University of Oxford and co-author of the study. “The Kheseen microfossils are just as well-preserved, yet animals continue to be absent — despite the fact we know at that point they existed.”

With fossil evidence proving elusive, the researchers turned to molecular clocks, a method that estimates when evolutionary lineages diverged by comparing genetic differences among living organisms and applying rates of genetic change. When constrained by the younger Weng’an deposits, the estimated origin of animals was comparatively recent. However, by incorporating older geological constraints from rock formations in Norway, Australia, and Arizona—deposited between 850 million and 730 million years ago—the timeline shifted significantly.

The adjusted analysis moved the estimated origin of animals back by approximately 200 million years, to between 800 million and 700 million years ago. “Pre-Ediacaran animal body fossils still elude us, and this analysis does not prove that animals existed 800 million years ago,” Durbin noted. “Meanwhile, our molecular-clock analyses show how much further back their evolutionary history could extend.”

The potential existence of animals during this earlier period coincides with the Cryogenian period, which began around 720 million years ago and featured extreme glaciations known as “Snowball Earth.” While biomarkers have previously suggested sponges may have lived as early as 650 million years ago, the new findings suggest an even longer history. The researchers plan to continue searching Neoproterozoic rocks with varying environmental conditions for further evidence.