Animals first occur in the fossil record around 574 million years ago. Their arrival appears as a sudden "explosion" in rocks from the Cambrian period (539 million years ago to 485 million years ago) and seems to counter the typically gradual pace of evolutionary change. Many scientists (including Darwin himself) believe that the first animals actually evolved long before the Cambrian period, but they cannot explain why they are missing from the fossil record.

The "molecular clock" method, for instance, suggests that animals first evolved 800 million years ago, during the early part of the Neoproterozoic era (1,000 million years ago to 539 million years ago). This approach uses the rates at which genes accumulate mutations to determine the point in time when two or more living species last shared a common ancestor. But although rocks from the early Neoproterozoic contain fossil microorganisms, such as bacteria and protists, no animal fossils have been found.

This posed a dilemma for paleontologists: does the molecular clock method overestimate the point at which animals first evolved? Or were animals present during the early Neoproterozoic, but too soft and fragile to be preserved?

To investigate this, a team of researchers led by Dr. Ross Anderson from the University of Oxford's Department of Earth Sciences have carried out the most thorough assessment to date of the preservation conditions that would be expected to capture the earliest animal fossils.

Lead author Dr. Ross Anderson said, "The first animals presumably lacked mineral-based shells or skeletons, and would have required exceptional conditions to be fossilized. But certain Cambrian mudstone deposits demonstrate exceptional preservation, even of soft and fragile animal tissues. We reasoned that if these conditions, known as Burgess Shale-Type (BST) preservation, also occurred in Neoproterozoic rocks, then a lack of fossils would suggest a real absence of animals at that time."