Animal origins may date back 800 million years, new analysis suggests
Fossils from Mongolia challenge an assumption used to date the first animals. The revised estimate reaches back before or into the Snowball Earth ice ages, but direct fossil proof remains elusive.
Animals may have originated between 800 million and 700 million years ago, according to a study published in Science Advances on 2 October 2026. The estimate pushes their possible beginnings roughly 200 million years earlier than estimates based on one Chinese fossil deposit. It could place the first animals before or during the severe ice ages known as Snowball Earth, although the researchers have not found animal body fossils from that early period.
The change rests on how scientists use the absence of fossils to set an upper age limit. At the roughly 590-million-year-old Weng’an Biota in China, microscopic organisms are preserved in exceptional detail, yet none has been definitively identified as an animal. Researchers had reasoned that animals probably evolved later: if they had already existed, the deposit might have preserved them. The new study tests that reasoning against a younger deposit in Mongolia.
Why Mongolian fossils changed the calculation
The research team examined more than 140 samples from the Kheseen Biota, including material from previously undocumented locations, using scanning electron microscopy. Kheseen is more than 40 million years younger than Weng’an. Its fossiliferous interval dates from after 550 million years ago to before 526 million years ago, according to the study.
Kheseen preserves intricate microfossils, including tiny organisms called acritarchs and embryo-like fossils with internal cells. None can confidently be identified as an animal. Yet animals are known from other places by the time the Kheseen deposits formed. Their absence from this well-preserved collection shows that a deposit can fail to capture animals even when animals exist elsewhere.
That finding weakens the case for treating the absence of animals at Weng’an as proof that they had not yet evolved. The authors suggest that early animals might have lived in different environments from those represented at Kheseen, or that local chemical conditions failed to preserve their remains. Neither explanation establishes exactly when animals first appeared; both show why an empty place in the fossil record needs careful interpretation.
How the estimate moved to 800–700 million years ago
The researchers instead considered older fossil-rich deposits as limits for their molecular-clock analysis: the Svanbergfjellet Formation in Norway, the Bitter Springs Group in Australia and the Chuar Group in Arizona. These deposits date from roughly 850 million to 730 million years ago. No confirmed animal fossils have been found in them, so they provide constraints for an estimate rather than direct evidence of animals living there.
A molecular clock combines genetic differences among living species with fossil dates and estimated rates of evolutionary change to infer when their ancestors may have lived. When the team used the older deposits instead of Weng’an as maximum-age constraints, its estimated origin for animals moved back to about 800 million to 700 million years ago. The result illustrates how strongly a date inferred by this method can depend on the fossil sites chosen to calibrate it.
Other evidence already points to a history older than the clearest animal body fossils. The paper says lipid biomarkers suggest an origin before 635 million years ago, while the body-fossil record reaches back to about 574 million years ago. Oxford’s account says chemical traces are consistent with sponges living at least 650 million years ago. Such traces offer indirect evidence; they are not confirmed animal bodies from the newly estimated 800-million-to-700-million-year interval.
What remains unknown about the first animals
If animals arose within the revised range, they may have preceded or lived through part of the Cryogenian Period, which began around 720 million years ago and included major glaciations. That timing raises a question about early animal life during Snowball Earth. It does not show that glaciation caused animals to evolve, or establish whether their origin came before or during those ice ages.
The first animals may have been small and soft-bodied, making them difficult to preserve without the hard structures found in many later animals. Preservation would also have depended on where they lived and the conditions after they died. First author Orin Lole Durbin said the analysis does not prove that animals existed 800 million years ago; it challenges an argument for confining their origin to the Ediacaran. Senior author Ross Anderson said the precise date remains uncertain.
The researchers call for searches across more locations, environments and preservation conditions. They say body fossils, traces of animal activity and chemical biomarkers should be assessed together. A confirmed early animal fossil would provide a more direct test of the revised timeline; until then, the 800-million-to-700-million-year range remains an estimate of when animals could have emerged.
Sources and context
- Re-evaluating molecular clock maximum age calibrations revives pre-Ediacaran divergence estimates for animalsScience Advances / PubMed record
- New study suggests animals may have evolved vastly earlier than fossil evidence suggestsUniversity of Oxford
- Animals may have evolved 200 million years earlier than we thoughtScienceDaily (University of Oxford materials)
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