Oxford study says animals may have evolved 200 million years earlier

Animal Origins Shift Backward as Microfossil Assumptions Fall

Molecular clock analyses push the origin of living animals back to roughly 800 to 700 million years ago, challenging long-held assumptions about fossil preservation. An Oxford-led study published in Science Advances reopens the debate on whether animal life emerged prior to Snowball Earth glaciations, relying on microfossils from Mongolia and older geological constraints to reshape the early evolutionary timeline.

Molecular Clocks Alter the Evolutionary Timeline

Scientists rely on molecular clocks to estimate when animal lineages first branched, combining genetic differences among living species with evolutionary-rate models and dates from the rock record. Securely identified fossils provide minimum age limits for these lineages, but establishing maximum age limits has historically depended on the absence of animal fossils in older, exquisitely preserved deposits. China’s roughly 590-million-year-old Weng’an Biota previously served as an upper age constraint because its microscopic organisms show no definitive animal remains. The central assumption held that exceptional preservation meant any existing animals would have left traces.

The Oxford research team tested that logic by examining the Kheseen Biota in Mongolia, a younger deposit aged between roughly 534 and 526 million years old. Scanning electron microscopy revealed 16 acritarch species and delicate embryo-like forms with internal cellular structures, yet none could be confidently 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 Ross Anderson.

Older Geological Constraints Replace Younger Limits

With the Weng’an absence argument weakened, the researchers turned to older candidate calibrations from the Tonian Period, including Norway’s Svanbergfjellet Formation, Australia’s Bitter Springs Group, and Arizona’s Chuar Group. These fossil-rich deposits date broadly to between 850 and 730 million years ago. Using these older rock formations as maximum age constraints shifted the estimated common ancestor of living animals backward by approximately 200 million years, landing between 800 and 700 million years ago.

This revised timeline overlaps directly with the Cryogenian Period, which began around 720 million years ago and featured extreme global glaciations known as Snowball Earth. While chemical fossils and biomarkers preserved in ancient rocks have previously pointed toward sponges living at least 650 million years ago, the new molecular clock estimates keep open the question of whether extreme ice ages influenced early animal evolution. Pre-Ediacaran body fossils still elude us, and this analysis does not prove that animals existed 800 million years ago, said first author Orin Lole Durbin, emphasizing that future discoveries across diverse environments will be necessary to pinpoint the exact birth date of the animal kingdom.