Molecular clocks push animal origins back 200 million years before first fossils

Animals may have emerged as much as 200 million years earlier than their first unmistakable appearance in the fossil record, according to a new study led by the University of Oxford and published in Science Advances. The research challenges a long-standing assumption that has anchored the dating of animal origins to the Ediacaran interval, and instead points to a birth date for the animal kingdom buried deep in the Neoproterozoic Era, potentially before some of the most extreme glaciations Earth has ever experienced. If the revised timeline holds, the evolutionary story of animals would stretch back to between roughly 800 and 700 million years ago, a period far removed from the Cambrian explosion that has long dominated textbook narratives of animal history.

The mystery at the centre of the study is a familiar one to palaeontologists. Animals appear relatively suddenly in the fossil record just before the Cambrian Period, which ran from 539 to 487 million years ago, yet a growing body of indirect evidence suggests their evolutionary history began much earlier. Molecular clock analyses, which compare genetic differences between living species and work backwards using fossil dates as calibration points, have repeatedly hinted at a deep pre-Ediacaran origin. But many recent studies have constrained those estimates using a seemingly straightforward argument drawn from the fossil record itself, and it is precisely that argument which the new work dismantles.

The argument hinges on an exceptionally well-preserved fossil deposit in China known as the Weng’an Biota, found in rocks of Ediacaran age and dated to roughly 590 million years ago. The Weng’an Biota preserves microscopic organisms in extraordinary cellular detail, yet none of the fossils it contains can be definitively identified as animals. Researchers have therefore inferred that the first animals must have originated after Weng’an, reasoning that a deposit capable of preserving such delicate structures would certainly have captured animal embryos or larvae if animals existed at the time. In effect, Weng’an has been treated as a maximum age constraint, a hard ceiling on how old animals could possibly be.

The Oxford-led team, which included researchers from the University of California Berkeley, ETH Zürich and Yale University, tested that logic by surveying a younger deposit with comparable preservation quality: the Kheseen Biota of Mongolia. The Kheseen Biota is more than 40 million years younger than Weng’an, yet the two deposits share some species, and by the time Kheseen was laid down animal fossils were already known from sites elsewhere in the world, including Namibia and South China. The researchers examined more than 140 samples, some from previously undocumented localities, using scanning electron microscopy to resolve fine anatomical detail at the microscopic scale.

The results were striking. The Mongolian material yielded exquisite new microfossil species, including acritarchs, tiny spherical organisms adorned with spines and branching projections, as well as embryo-like fossils preserving internal cells. The quality of preservation rivals anything seen in the Ediacaran record. Yet despite this remarkable fidelity, not a single specimen could be confidently assigned to the animal kingdom. Animals were demonstrably alive when the Kheseen Biota formed, but they left no trace within it, a direct contradiction of the preservation-based reasoning applied to Weng’an.

Senior author Associate Professor Ross Anderson of the Museum of Natural History at Oxford University explained the significance of the finding. The Kheseen Biota, he said, breaks the argument that the exceptional microfossils of Weng’an mean animal fossils would have been seen in the assemblage had they existed at the time. The Kheseen microfossils are just as well-preserved, yet animals continue to be absent, despite the fact that they are known to have existed by that point. The most plausible explanation is that early animals lived in environments different from those that preserved the Kheseen fossils, or that the local chemical conditions after death simply failed to preserve animal remains.

With the Weng’an ceiling removed, the team rebuilt their molecular clock analyses using much older geological constraints. They turned to fossil-rich deposits dating from roughly 850 to 730 million years ago, including the Svanbergfjellet Formation in Norway, the Bitter Springs Group in Australia and the Chuar Group in Arizona. These deposits have the potential to preserve animals even though definitive animal fossils have eluded discovery within them, and they have previously served as maximum dates for animal origins in other studies. When the molecular clock was anchored to these older formations rather than to the Ediacaran Weng’an deposit, the estimated origin of animals shifted backwards by around 200 million years, landing between 800 and 700 million years ago.

That timeline is not without independent support. Chemical fossils, known as biomarkers, preserved in ancient rocks provide evidence consistent with sponges living at least 650 million years ago, tens of millions of years before the oldest definitive macroscopic animal fossils appear. The molecular clock estimate also raises a provocative possibility: animals may already have evolved before Earth entered the Cryogenian glaciations, or even emerged during that inhospitable interval. The Cryogenian Period began around 720 million years ago, when enormous glaciers spread across the planet in episodes commonly known as Snowball Earth. The new study reopens the question of whether the dramatic environmental conditions associated with those global ice ages played a role in the origin and early evolution of animal life, a possibility that had been closed off under the younger Ediacaran timeline.

First author Orin Lole Durbin, who conducted the work as an undergraduate at Oxford and is now at Virginia Tech, cautioned that the analysis does not prove animals existed 800 million years ago. Pre-Ediacaran animal body fossils still elude researchers, and the molecular clock provides an estimate rather than a direct observation. However, the new fossil evidence from Mongolia undermines one of the main arguments for restricting animal origins to the Ediacaran interval, while the molecular-clock analyses show how much further back their evolutionary history could extend. The distinction matters: the study does not claim to have found ancient animals, but it removes a key obstacle to the idea that they existed far earlier than the rocks currently reveal.

Part of the reason early animals remain so elusive lies in their biology. The first animals would have been small and soft-bodied, lacking the shells, bones and other hard structures that make later animals so much easier to fossilise. Whether such creatures were preserved at all depended on their environment and on an unusual combination of chemical conditions after death, conditions that evidently failed in the Kheseen setting even as exquisite microfossils of other organisms formed. The researchers argue that future work should therefore explore fossil deposits from different parts of the world, representing different environments and modes of fossilisation, in the search for animals. Such searches should also weigh all available lines of evidence, including body fossils, traces of animal activity and chemical biomarkers, rather than relying on any single signal. Until that evidence becomes available, as Anderson noted, the precise birth date of the animal kingdom remains uncertain, but the window of possibility has now swung dramatically open toward the deep Neoproterozoic.

Subject of Research: Re-evaluating fossil calibration constraints on the molecular clock timing of animal origins

Article Title: New study suggests animals may have evolved vastly earlier than fossil evidence suggests

Article References: New study suggests animals may have evolved vastly earlier than fossil evidence suggests. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: animal origins, molecular clock, Ediacaran, Neoproterozoic, Weng'an Biota, Kheseen Biota, Snowball Earth, Cryogenian, microfossils, sponges, Cambrian explosion, fossil record