Yet despite this remarkable preservation, none of the fossils can be confidently identified as an animal.

"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 senior author Ross Anderson, an associate professor at the Oxford University Museum of Natural History. "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 implication is striking: either animals lived in different environments from those that preserved the Kheseen fossils, or the chemical conditions there simply failed to preserve animal remains. If a pristine deposit can miss animals that definitely existed, then Weng'an's silence no longer proves animals were absent 590 million years ago.

Rewinding the molecular clock

Freed from that constraint, the researchers looked much further back—to fossil-rich deposits between roughly 850 and 730 million years old, including the Svanbergfjellet Formation in Norway, the Bitter Springs Group in Australia and the Chuar Group in Arizona, United States. All have the potential to preserve animals, and all have previously served as maximum dates for animal origins.

Using these older constraints, the team applied a technique called molecular clock analysis. This method compares genetic differences between living species and, using fossil dates as reference points combined with estimated rates of evolution, works backward to estimate when their common ancestors lived.

Compared with analyses anchored to the younger Weng'an deposit, this approach pushed the estimated origin of animals back by around 200 million years—to somewhere between 800 and 700 million years ago.

Independent clues already point in that direction. Chemical fossils, or biomarkers, preserved in ancient rocks provide evidence consistent with sponges living at least 650 million years ago, tens of millions of years before the oldest definitive macroscopic animal fossils.

These early animals would have been small and soft-bodied, lacking the shells, bones and other hard structures that make their later relatives easier to fossilize. Whether such creatures left any record depended heavily on their environment and on an unusual combination of chemical conditions after death.

Life before Snowball Earth?

An origin between 800 and 700 million years ago raises a striking possibility: animals may already have evolved before Earth entered the Cryogenian Period, which began around 720 million years ago. During that time, enormous glaciers spread across the planet in episodes commonly known as Snowball Earth. The study reopens questions about whether conditions associated with these extreme ice ages played any role in the origin and early evolution of animal life.

The researchers urge caution, though. "Pre-Ediacaran animal body fossils still elude us, and this analysis does not prove that animals existed 800 million years ago," said first author Orin Lole Durbin, who was an Oxford undergraduate during the study and is now at Virginia Tech. "However, our new fossil evidence from Mongolia undermines one of the main arguments for restricting animal origins to the Ediacaran interval. Meanwhile, our molecular-clock analyses show how much further back their evolutionary history could extend."

Future work, the team says, should examine fossil deposits from different parts of the world, representing different environments and modes of fossilization. The search should also weigh all available evidence for animals, including body fossils, traces of their activity and chemical biomarkers.

"Until that evidence becomes available, the precise birth date of the animal kingdom remains uncertain," Anderson added.

The study is published as Orin Lole Durbin et al., "Re-evaluating molecular clock maximum age calibrations revives pre-Ediacaran divergence estimates for animals," Science Advances (2026), DOI: 10.1126/sciadv.aeg6289.