A study by the University of Oxford suggests that the first animals appeared on Earth 800–700 million years ago — long before the period traditionally considered the beginning of faunal history. Scientists reached this conclusion after reassessing the role of fossil finds and applying molecular clock analysis.

The absence of fossils turned out not to be proof that animals were absent

For a long time, it was believed that animals appeared around 590 million years ago. One basis for this estimate was the rocks of China’s Weng’an Formation, where microscopic organisms were preserved but no animals were found.

The logic seemed convincing: if ancient organisms were well preserved but there were no traces of animals among them, then animals must not have existed yet at that time.

However, the researchers found a problem with this approach. They studied the Kheseen Formation in Mongolia, which is about 40 million years younger than Weng’an and is distinguished by similarly excellent preservation of organic remains. No animal fossils have been found there either — although it is known that by that time animals already existed in other regions.

The Mongolian formation thus became a kind of control example. It shows that even exceptionally good conditions for preserving ancient organisms do not guarantee that animals will necessarily leave fossils behind.

Molecular clocks looked deeper into the past

After that, scientists reconsidered the significance of even older rocks dating back 850–730 million years, found in Norway, Australia, and the United States. The absence of animal fossils in them could no longer be regarded as evidence that animals did not exist at that time.

Therefore, the researchers turned to another method — molecular clocks. This approach makes it possible to estimate the time when evolutionary lineages diverged based on genetic differences between modern organisms.

Calculations showed that the common ancestors of animals may have existed about 800–700 million years ago. This pushes the presumed origin of fauna back by about 200 million years compared with the traditional estimate.

The first animals left almost no traces

The reason animals appeared so much later in the fossil record may be related to what they were like.

The first animals were probably tiny soft-bodied organisms. They had no shells, bones, or other hard structures that fossilize easily. After death, such organisms could disappear completely if environmental conditions did not favor the preservation of their tissues.

Therefore, there may be an enormous span of time between the actual appearance of animals and the first reliable fossil evidence of them.

Animals may have survived “Snowball Earth”

If the calculations are correct, the ancestors of modern animals already existed 800–700 million years ago — that is, before or during the Cryogenian Period.

This is especially interesting because it was precisely then that Earth was going through one of the harshest stages in its history. The planet repeatedly experienced global glaciations known as “Snowball Earth,” when vast ice sheets spread across a significant part of its surface.

Thus, the early history of animals may have begun much earlier than the fossil record suggested and may have included the existence of the first representatives of fauna in a world that would soon face near-global glaciation.