Research led by the University of Oxford suggests that the earliest animals may have existed hundreds of millions of years before the first unmistakable animal fossils appeared. The findings, published on October 2, indicate that animals may have originated deep in the Neoproterozoic Era—possibly before the most severe ice age in Earth’s history.

The research could reshape scientists’ understanding of a major evolutionary puzzle. Animals seem to appear relatively suddenly in the fossil record just before the Cambrian Period, which lasted from about 539 million to 487 million years ago. However, other lines of evidence have long suggested that their evolutionary history began much earlier.

The new study focuses on how researchers have used ancient fossil deposits to establish the maximum possible age of animal life.

Rethinking assumptions about the oldest animal fossils

One important site is the Wen’an biota in China, an approximately 590-million-year-old fossil deposit preserved in Ediacaran-era rocks. The site contains exceptionally well-preserved microorganisms, but none can be clearly identified as animals.

Because animals are absent from the deposit, scientists have previously argued that they probably evolved after the Wen’an biota formed. The reasoning was that if animals had already existed, such an unusually well-preserved fossil deposit should have captured evidence of them.

A new study challenges that assumption.

Mongolian fossils test the fossil-record argument

An international team that included scientists from the University of California, Berkeley, ETH Zurich, and Yale University conducted an extensive study of exceptionally preserved microfossils in Mongolia’s Kessien biota.

Although the Kessien biota is more than 40 million years younger than Wen’an, the two fossil assemblages share several species. Importantly, when the Kessien biota formed, animals were already known from other parts of the world, including sites in Namibia and southern China.

Researchers used scanning electron microscopy to examine more than 140 samples, including material collected from previously undocumented locations. Their analysis revealed new, exquisitely preserved microfossil species, including acritarchs—small spherical fossils with spines and branching processes—as well as embryo-like fossils containing internal cells.

Despite their remarkable preservation, none of the fossils could be confidently identified as animals.

Lead author Associate Professor Ross Anderson of the Natural History Museum at Oxford University said:

“The Kessien biota dispels the argument that Wen’an’s exceptional microfossils, if they existed at the time, meant a collection of animal fossils. The Kessien microfossils are similarly well preserved, but animals continue to disappear, even though we knew they existed at the time.”

This suggests that animals may have lived in environments different from those represented by the Kessien deposits. Alternatively, the chemical conditions responsible for fossilization may not have been suitable for preserving animal remains.

Animal life may be hidden in the fossil record

Animals were already present when the Kessien biota formed, but they do not appear in those fossils. The researchers therefore argue that the absence of animals in Wen’an cannot automatically be treated as evidence that animals had not evolved 590 million years ago.

Instead, the research team examined much older fossil deposits dating from about 850 million to 730 million years ago. These include the Svanbergferret Formation in Norway, the Bitter Springs Group in Australia, and the Tuar Group Formation in Arizona, United States.

All three are considered fossil-rich deposits capable of preserving animals, yet no confirmed animal remains have been found in them. These sites have previously been used to define the maximum possible age of animal life.

Using these older geological constraints, the researchers performed a molecular clock analysis. This technique compares genetic differences between living species and combines them with fossil ages and estimated rates of evolutionary change to calculate when their common ancestor lived.

When the researchers used the older deposits as constraints instead of the younger Ediacaran Wen’an site, they pushed the probable origin of animals back by about 200 million years. The new estimate places the appearance of animals between approximately 800 million and 700 million years ago.

Biomarkers offer clues to ancient animals

Other evidence also suggests that animals may have a much deeper evolutionary history than body fossils indicate.

Chemical fossils, also known as biomarkers, preserved in ancient rocks contain evidence consistent with sponges that lived at least 650 million years ago. That would place them tens of millions of years before the earliest visible animal fossils.

Early animals were probably small and flexible. Without shells, bones, or other hard structures, their remains would have been less likely to survive fossilization than those of many later animals.

Their chances of preservation also likely depended heavily on where they lived and on the rare combination of chemical conditions present after death.

Did animals exist before Snowball Earth?

If animals originated between 800 million and 700 million years ago, they may have existed before Earth entered its most extreme ice age—or even appeared during it.

The Cryogenian Era began about 720 million years ago, when giant glaciers expanded across Earth during a phenomenon commonly known as Snowball Earth.

The findings renew questions about whether the environmental conditions associated with Snowball Earth influenced the origin and early evolution of animal life.

Lead author Orin Lorre Durbin, who was an undergraduate at the University of Oxford when the study was conducted and is now at Virginia Tech, said:

“Pre-Ediacaran animal fossil remains elude us, and this analysis suggests that animals existed 800 million years ago. However, the new fossil evidence from Mongolia undermines one of the main arguments that limits the origin of animals to the Ediacaran Period. Our molecular clock analysis, on the other hand, shows how far back their evolutionary history goes.”

Scientists continue the search for the oldest animals

The researchers say future studies should examine fossil deposits from a wider range of locations, environments, and preservation conditions in the search for evidence of early animals.

Future research will also need to combine all available evidence, including animal fossils, traces of animal activity, and chemical biomarkers.

Professor Anderson added:

“Until that evidence is available, the exact birth date of the animal kingdom will remain unknown.”