Open Access Article
Orin M. Lole Durbin et al., Re-evaluating molecular clock maximum age calibrations revives pre-Ediacaran divergence estimates for animals. Sci. Adv.12, eaeg6289(2026).DOI: 10.1126/sciadv.aeg6289
Snippet from press release:
Animals may have evolved up to 200 million years earlier than their first appearance in the fossil record, according to a new study led by the University of Oxford. The findings, published in Science Advances, challenge key assumptions scientists have used to date the origins of animal life.
The findings could transform our understanding of one of evolution’s greatest mysteries: why animals seem to appear relatively suddenly in the fossil record just before the Cambrian Period (539 to 487 million years ago), despite other evidence suggesting that their evolutionary history began much earlier.
At the heart of the study is a problem with the way scientists have recently used the fossil record to set a maximum age for the origin of animals.
A roughly 590-million-year-old fossil deposit known as the Weng’an Biota in rocks of Ediacaran age in China preserves microscopic organisms in extraordinary detail, none of which are definitive animals. Scientists have therefore inferred that the first animals must have originated after Weng’an, assuming that this exceptional fossil deposit would have preserved them if they existed at this time.
The research team, which included members from the University of California Berkeley, ETH Zürich, and Yale University, carried out a comprehensive survey of exceptionally well-preserved microfossils from the Kheseen Biota in Mongolia. The Kheseen Biota is more than 40 million years younger than Weng’an, even though they share some species. By this time animal fossils are known from sites elsewhere around the world, including Namibia and South China.
The researchers studied over 140 samples, including some from previously undocumented localities, and examined them using scanning electron microscopy. This revealed exquisite new microfossil species including acritarchs (tiny spherical organisms with spines and branching projections) as well as embryo-like fossils containing internal cells. Yet, despite this remarkable level of preservation, none can be confidently identified as an animal.
Animals must have lived in different environments from those that preserved the fossils in the Kheseen Biota, or the chemical conditions there failed to preserve animal remains.
“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. The Kheseen microfossils are just as well-preserved, yet animals continue to be absent – despite the fact we know at that point they existed.”
Given that the Kheseen Biota was preserved when animals were present but they are nonetheless not preserved as fossils, the researchers argue that Weng’an does not automatically qualify as evidence that no animals existed at that time and only appeared later than 590 million years ago.
Instead, they looked much further back, to several deposits dating from roughly 850 to 730 million years ago. These included the Svanbergfjellet Formation (Norway), Bitter Springs Group (Australia) and Chuar Group (Arizona, USA) - all fossil-rich deposits with the potential to preserve animals, even though they have eluded discovery. These deposits have previously been used as maximum dates for the origin of animals.
Using these older geological constraints, the researchers used a technique called molecular clock analysis. This method compares genetic differences between living species and, using dates from the fossil record as reference points combined with rates of evolution, works backwards to estimate when their ancestors lived. Compared with analyses constrained by the younger Ediacaran Weng’an deposit, this approach shifted the estimated origin of animals backwards by around 200 million years, to between 800 and 700 million years ago.