When did the first animals appear on Earth? If the fossil record is to be believed, the first animals evolved about 570m years ago and proliferated rapidly during the Cambrian explosion, approximately 539m to 485m years ago. But if you follow the molecular clock, which uses the rate at which genes accumulate mutations to extrapolate living animals back to their oldest common ancestor, animals probably first evolved about 800m years ago.
Now researchers have taken a different approach and asked when were conditions right for preserving animal fossils? The first animals are thought to have lacked mineral-based shells or skeletons, and would have needed exceptional conditions to be fossilised, akin to the famous 500m-year-old Burgess Shale deposits in the Canadian Rockies.
Analysis of Burgess Shale-like rocks revealed that certain types of clay increase the chances of soft-bodied fossil preservation. The researchers showed that only three rock deposits from 800m years ago met these criteria – in Nunavut in Canada, Siberia in Russia and Svalbard in Norway – but none contained animal fossils.
Writing in Trends in Ecology and Evolution, the researchers conclude that the earliest possible origin of animals was 789m years ago. Now they hope to refine their estimate even further by searching through progressively younger rocks.
