Animals may have evolved 200 million years earlier than thought

The first animals may have appeared up to 200 million years earlier than the fossil record suggests, according to a new study led by the University of Oxford.

The research, published in Science Advances, suggests animal life could have originated between 800 and 700 million years ago, deep in the Neoproterozoic Era and possibly before some of the most severe ice ages in Earth's history.

A puzzle in the rocks

Animals appear quite suddenly in the fossil record shortly before the Cambrian Period, which began around 539 million years ago. But other evidence has long suggested that their story began much earlier.

A key piece of evidence has been the Weng'an Biota in China, a deposit around 590 million years old in which microscopic organisms are preserved in exceptional detail. None of them can be definitely identified as an animal. Scientists have argued that if animals had existed then, such a well-preserved site would have captured them, so animals must have evolved later.

What the Mongolian fossils show

The team, which included scientists from the University of California, Berkeley, ETH Zurich and Yale University, studied exceptionally preserved microfossils from the Kheseen Biota in Mongolia. The Kheseen fossils are more than 40 million years younger than Weng'an, from a time when animals are known to have existed elsewhere in the world, including in Namibia and South China.

The researchers examined more than 140 samples with scanning electron microscopes and found new species preserved in exquisite detail, including spiny microscopic organisms and embryo-like fossils with internal cells. But none could be confidently identified as an animal.

"The Kheseen Biota breaks the argument that the exceptional microfossils of Weng'an mean we would have seen animal fossils in the assemblage had they existed at the time," said Associate Professor Ross Anderson, of Oxford University's Museum of Natural History, the study's senior author. "The Kheseen microfossils are just as well-preserved, yet animals continue to be absent, despite the fact we know at that point they existed."

Turning back the molecular clock

If the absence of animals at Weng'an does not prove they had not yet evolved, scientists need a different upper limit for when animals first appeared. The team used older fossil deposits, dating from around 850 to 730 million years ago, in Norway, Australia and Arizona.

They then carried out a molecular clock analysis, which compares genetic differences between living species with fossil dates and estimated rates of evolution to work out when common ancestors lived. Using the older deposits, the estimated origin of animals moved back by around 200 million years.

Supporting evidence

Chemical traces known as biomarkers, preserved in ancient rocks, already suggest that sponges were living at least 650 million years ago, tens of millions of years before the oldest definite animal fossils.

The earliest animals were probably small and soft-bodied, with no shells or bones, which would have made them very unlikely to be fossilised. Whether they were preserved would also have depended on where they lived and on a rare combination of chemical conditions after they died.

Surviving a frozen planet

If animals did originate 800 to 700 million years ago, they may already have existed before the "Snowball Earth" ice ages, when ice may have covered much or all of the planet. That raises the extraordinary possibility that early animals survived some of the harshest conditions in Earth's history, and that those conditions may even have shaped how they evolved.