When did animals first appear? The oldest clear animal fossils are about 574 million years old. Some scientists think animals are much older. A fossil site in China has many tiny fossils but no clear animals. So people thought animals came later.

Then a team studied a similar site in Mongolia. Animals were alive when those rocks formed, but the site had no clear animal fossils either. The team now says animals began about 800 to 700 million years ago.

Scientists can estimate when groups of living things split apart by comparing their DNA. This method is called a molecular clock, and it needs dates from fossils to set it. One important date is a maximum age, a point before which animals probably did not exist.

Some studies used the Weng'an Biota in China for this. Its tiny fossils are beautifully preserved, but none are clearly animals. A new study compared it with the Kheseen Biota in Mongolia, which formed between about 550 and 526 million years ago. Animals certainly lived then, yet no clear animals were found there either.

So an empty site cannot prove that animals were missing. Using other reference points, the team placed the origin of animals at about 800 to 700 million years ago.

The oldest animal body fossils are about 574 million years old. Chemical traces linked to sponges turn up in rocks older than 635 million years, and molecular clocks, which convert DNA differences into time, often point earlier still. To run such a clock, researchers need fossil dates that act as limits. A minimum age is easy to defend, since a fossil proves a group existed.

A maximum age is harder, because it rests on absence. Some analyses have used the Weng'an Biota of China, where tiny fossils are preserved in fine detail, on the assumption that animals would have shown up there had they existed. Orin Lole Durbin and colleagues tested that assumption against the Kheseen Biota in Mongolia.

Its preservation and its set of microfossils are similar, and it formed between about 550 and 526 million years ago, when animals were definitely around. It holds no definite animals either. The team prefers several sites older than the Ediacaran Period that keep both non-animal fossils and biomarkers, and their analyses support an origin about 800 to 700 million years ago. A 2024 study with different calibrations put the origin in the early Ediacaran instead.