Animals May Have Evolved Up to 200 Million Years Before the Fossil Record Shows, New Study Suggests
Exceptionally preserved Mongolian microfossils with no animals in them undercut a key argument for a late origin of animals, and molecular-clock analysis points to animal origins between 800 and 700 million years ago, Oxford-led researchers report.
The first animals may have evolved between 800 and 700 million years ago, roughly 200 million years earlier than some estimates based on the fossil record, according to a University of Oxford-led study published October 2 in Science Advances and reported by EurekAlert.
A fossil site that should have contained animals, and did not
For years, scientists pointed to the 590-million-year-old Weng'an Biota as evidence that animals came later: its microfossils are so well preserved that animals, had they existed, should have turned up. The new study examined the Kheseen Biota in Mongolia, which is about 550 million years old and preserved just as well, using scanning electron microscopy. It contains spiny microfossils called acritarchs and embryo-like fossils with internal cells, but no confidently identified animals, even though animal remains are known from other sites of the same age, including in Namibia and South China, according to Phys.org.
“The Kheseen Biota breaks the argument that exceptional microfossils of Weng'an mean we would have seen animal fossils had they existed.” — Ross Anderson, University of Oxford
Molecular clocks point further back
The team combined the fossil evidence with a molecular clock analysis, which uses genetic differences between living species and fossil dates as reference points to estimate when lineages split. That analysis places the origin of animals between 800 and 700 million years ago, before or during the Snowball Earth glaciations that began around 720 million years ago. PhD student Orin Lole Durbin said the new fossil evidence “undermines one of the main arguments for restricting animal origins to the Ediacaran interval.”
Why the answer is still open
The study does not prove that animals existed 800 million years ago. Early animals were probably small and soft-bodied, which makes them hard to preserve, and no fossil has yet been found from that earlier window. In a commentary published by Yale News, paleontologist Derek Briggs noted that the oldest known animal fossils date to 574 million years ago and said future discoveries are needed to refine when animal life began. Anderson put it plainly: “Until that evidence becomes available, the precise birth date of the animal kingdom remains uncertain.”
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