Study pushes possible origin of animals back to 800–700 million years ago — ScienceDaily

An international group of scientists led by the University of Oxford suggested that the first animals may have appeared 800–700 million years ago — approximately 200 million years earlier than some previous estimates indicated. The study was published in the journal Science Advances, ScienceDaily reports.

Why the boundary was revised

The researchers questioned the use of the Weng'an biota in China, about 590 million years old, as an upper limit for the time of animal origins. This fossil community preserved microorganisms in high detail, but none of the specimens can be confidently classified as animals. Previously, the absence of such remains was considered an argument that animals emerged after these deposits formed.

The team studied more than 140 microfossil specimens from the Heseen biota in Mongolia, which is more than 40 million years younger than Weng'an. By the time the Mongolian deposits formed, animals were already known from other locations, including Namibia and South China. At the same time, the researchers also found no remains that could be reliably identified as animals in the well-preserved Heseen specimens.

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Molecular clock analysis

The scientists suggested that animals may have lived in environments different from those represented in the Heseen deposits, or that fossilization conditions there did not favor the preservation of their remains. Therefore, the absence of animals in Weng'an cannot automatically mean that they did not exist 590 million years ago.

For molecular clock analysis, the team used older fossil deposits approximately 850–730 million years old: the Svanbergfjellet Formation in Norway, the Bitter Springs Group in Australia, and the Chuar Group in Arizona. After changing the geological constraints, the estimate for the time of the appearance of animals shifted about 200 million years further into the past.

The authors emphasize that the analysis does not prove the existence of animals 800 million years ago, since pre-Ediacaran animal fossils have still not been found. At the same time, chemical biomarkers in ancient rocks contain evidence consistent with the existence of sponges at least 650 million years ago. According to the new estimate, early animals may have existed before or during the Cryogenian period, when extensive glaciations known as Snowball Earth began around 720 million years ago.