Oxford-led study says animals may have evolved 200 million years earlier than thought

New research suggests animals may have evolved up to 200 million years before they first appear clearly in the fossil record, according to a study published Oct. 2 in Science Advances. The study was led by the University of Oxford and included scientists from the University of California Berkeley, ETH Zürich and Yale University. The findings indicate animals could have originated deep in the Neoproterozoic Era, possibly before some of the most severe ice ages in Earth's history, according to the researchers.

The findings could reshape scientists' understanding of a major evolutionary puzzle. Animals appear relatively abruptly shortly before the Cambrian Period, which ran from 539 to 487 million years ago, even though other lines of evidence have long suggested their evolutionary story began much earlier. One key site is the Weng'an Biota in China, a roughly 590-million-year-old deposit of Ediacaran age that preserves microscopic organisms in exceptional detail but contains nothing definitively identified as an animal. Scientists previously argued that animals likely evolved after Weng'an formed, reasoning that a well-preserved deposit should have captured evidence of them if they had existed.

The team studied exceptionally preserved microfossils from the Kheseen Biota in Mongolia. The Kheseen Biota is more than 40 million years younger than Weng'an, although the two assemblages share some species. Animals are already known from other locations around the world by the time Kheseen formed, including sites in Namibia and South China.

Researchers examined more than 140 samples, including material from previously undocumented locations, using scanning electron microscopy. Their analysis revealed new microfossil species, including acritarchs — tiny spherical organisms with spines and branching projections — along with embryo-like fossils containing internal cells. None of the fossils can confidently be identified as an animal.

Senior author Associate Professor Ross Anderson, of the Museum of Natural History at Oxford University, said: "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. The Kheseen microfossils are just as well-preserved, yet animals continue to be absent — despite the fact we know at that point they existed."

The researchers suggest animals may have occupied environments different from those represented in the Kheseen deposits, or that the chemical conditions responsible for fossilization there were not suitable for preserving animal remains. Because animals were already present when the Kheseen Biota formed but still do not appear in its fossil record, the team argues that the absence of animals from Weng'an cannot automatically be taken as proof that animals had not yet evolved 590 million years ago.