Animal origins may be 200 million years older, Oxford-led study suggests

A study published Oct. 2 in Science Advances by researchers at the University of Oxford and other institutions suggests that the earliest animals may have appeared hundreds of millions of years earlier than the oldest known animal fossils indicate. The findings, based on exceptionally preserved microfossils from Mongolia and molecular clock calculations, could push the origin of animal life back to between 800 and 700 million years ago.

The fossil record shows a relatively sudden appearance of animal fossils shortly before the Cambrian Period (539 to 487 million years ago). But other evidence, including genetic data, has long hinted that animals emerged much earlier. The new study addresses a key argument that kept the origin date at around 590 million years ago.

The Weng'an assumption fails a test

Scientists previously used the Weng'an Biota in China, a 590 million year old fossil deposit with exquisite preservation, to set a maximum age for animal origins. Because no animals were found there, the reasoning went, animals must have evolved after that deposit formed. The logic assumed that if animals existed, such well-preserved rocks would have caught them.

The new study tested that assumption using the Kheseen Biota in Mongolia, a deposit more than 40 million years younger. The team examined more than 140 samples, including material from previously undocumented locations, using scanning electron microscopy. They found new microfossil species, including acritarchs and embryo-like structures, all beautifully preserved. Yet again, no fossils could confidently be identified as animals.

"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 senior author Associate Professor Ross Anderson of Oxford's Museum of Natural History. "The Kheseen microfossils are just as well-preserved, yet animals continue to be absent, despite the fact we know at that point they existed."

Since animals were already present when Kheseen formed (they're known from places like Namibia and South China), their absence in the deposit suggests that some environments or preservation conditions simply don't capture animal remains. That undermines the earlier reasoning about Weng'an.

Molecular clock pushes back the timeline

With the Weng'an constraint weakened, the researchers turned to older fossil deposits from Norway, Australia and Arizona, dating between 850 and 730 million years ago. Those sites also lack confirmed animal fossils but were considered capable of preserving them. Using those older geological constraints in molecular clock analyses (which compare genetic differences between living species and combine them with fossil dates to estimate when common ancestors lived) the estimated origin of animals moved back by roughly 200 million years.

The new range places animal origins between 800 and 700 million years ago. That interval coincides with or predates the Cryogenian Period (starting around 720 million years ago), when massive glaciers covered much of the planet during episodes known as Snowball Earth.

Chemical fossils from ancient rocks have already hinted at sponges living at least 650 million years ago, tens of millions of years before the oldest definitive macroscopic animal fossils. The earliest animals were likely small, soft bodied, and rarely preserved.

Still searching for direct evidence

First author Orin Lole Durbin, a University of Oxford undergraduate at the time of the study now at Virginia Tech, cautioned: "Pre-Ediacaran animal body fossils still elude us, and this analysis does not prove that animals existed 800 million years ago. However, our new fossil evidence from Mongolia undermines one of the main arguments for restricting animal origins to the Ediacaran interval. Meanwhile, our molecular-clock analyses show how much further back their evolutionary history could extend."

The researchers emphasize that future studies must examine fossil deposits from more diverse locations, environments, and preservation conditions, and combine body fossils, trace fossils, and chemical biomarkers. Until such evidence emerges, the precise birth date of the animal kingdom remains uncertain.

• molecular clock
• Neoproterozoic
• paleontology
• Science Advances
• Snowball Earth
• University of Oxford