Animals may be up to 200 million years older than the fossils say

A team led from the University of Oxford says animals may have appeared between 800 and 700 million years ago, up to 200 million years before the oldest accepted fossil evidence. The claim, published in Science Advances on October 2, rests on a Mongolian fossil site that looks perfect for preserving animals and yet appears to contain none.

A fossil bed that should have shown animals

The site is called the Kheseen Biota. Its microfossils are about 550 million years old and are exceptionally well preserved. According to the researchers, they do not include definitive animal remains, even though animals were already living elsewhere at that time.

That matters because of an older argument. The Weng'an fossil deposit in China is about 590 million years old and also preserves tiny organisms in fine detail. For years, its lack of animals was read as a sign that animals simply had not evolved yet.

The Kheseen result cuts against that logic. If a fossil bed from 550 million years ago can be this well preserved and still show no animals, then a missing animal in a beautiful fossil bed does not prove that animals were absent from the world.

Senior author Ross Anderson of Oxford's Museum of Natural History put it this way: "The Kheseen Biota breaks the argument that exceptional microfossils would show animal fossils if they existed at that time." The first author is Orin Lole Durbin, an Oxford PhD student who is now at Virginia Tech.

What the molecular clock adds

The team combined the fossil argument with a molecular clock analysis. The approach counts genetic differences among species alive today and works backward to estimate when their lineages diverged.

Using it, the researchers estimate that animals originated between 800 and 700 million years ago. The accepted fossil record puts the first animals roughly 200 million years later.

Small, soft and easy to miss

If the estimate holds, the earliest animals were probably small and soft-bodied. According to the release, such creatures would only fossilize under specific environmental and chemical conditions, which would explain why the record is so thin.

The authors are not claiming to have found an animal from 800 million years ago. They are arguing that the absence of one in the rock record is weaker evidence than it looked.

Molecular clocks come with their own uncertainty, and the release does not give error ranges. The open question is whether older fossil sites, if any exist and are preserved the right way, will ever show what the genes seem to imply.