Animals May Have Evolved 200 Million Years Earlier Than Fossils Suggest
Animals may have evolved hundreds of millions of years earlier than their clearest fossils suggest, according to a new study led by the University of Oxford.
Published on 2 October in Science Advances, the research challenges a long-standing assumption used to estimate when the animal kingdom first emerged. Fossils showing recognisable animals become common shortly before the Cambrian Period, which began about 539 million years ago, but genetic evidence has often pointed to a much older origin.
The problem has been the apparent absence of animals from exceptionally preserved older fossil deposits. One of the most important is the roughly 590-million-year-old Weng’an Biota in China. Its microscopic fossils are preserved in extraordinary detail, yet none can be confidently identified as an animal. Researchers have therefore used the site as evidence that animals had not evolved by that time.
The new study puts that reasoning under pressure.
An international team including scientists from Oxford, the University of California, Berkeley, ETH Zürich and Yale University examined more than 140 samples from the Kheseen Biota in Mongolia. Using scanning electron microscopy, they found exceptionally preserved microfossils, including acritarchs and embryo-like fossils containing internal cells.
Yet animals were absent there too, even though other fossil sites show that animals already existed when the Kheseen deposits formed.
Associate Professor Ross Anderson of Oxford’s Museum of Natural History said the findings undermine the idea that exceptionally preserved deposits should necessarily contain animals if they were alive at the time.
The researchers then reconsidered older fossil deposits from around 850 to 730 million years ago, including formations in Norway, Australia and Arizona. Using these geological constraints in molecular-clock analyses shifted the estimated origin of animals back by around 200 million years, to roughly 800–700 million years ago.
That possibility is significant because it places the potential emergence of animals before or during the Cryogenian Period, when Earth experienced the extreme glaciations associated with “Snowball Earth”.
The study does not prove that animals existed 800 million years ago. Instead, it shows that their absence from particular fossil deposits cannot reliably establish when they evolved. Early animals were probably small and soft-bodied, making preservation highly dependent on their environment and local chemistry.
As first author Orin Lole Durbin noted, the precise birth date of the animal kingdom remains unresolved. The search will now need to bring together body fossils, traces of animal activity and chemical biomarkers from a much wider range of ancient environments.
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