Researchers using molecular clock analysis suggest that the first animal life on Earth may have originated between 800 million and 700 million years ago, pushing the evolutionary timeline back by up to 200 million years beyond the oldest known body fossils.
The earliest undisputed fossil evidence of animals dates to roughly 574 million years ago during the Ediacaran period. A new study published in the journal Science Advances argues that the absence of animal fossils in ancient rocks does not prove animals were missing from the planet at the time. Researchers suggest that environmental conditions or unfavorable fossilization processes may have left a key chapter of early evolution out of the physical record.
Researchers Test Assumptions About Early Animal Evolution
For years, scientists relied on exceptionally preserved deposits like the roughly 590-million-year-old Weng’an Biota in China to set an upper limit for when animals could have evolved. Because these rock layers preserve microscopic organisms in extraordinary detail without containing definitive animal fossils, researchers previously inferred that animals simply had not yet appeared.
To test that assumption, an international research team examined more than 140 samples from the Kheseen Biota in Mongolia, which is more than 40 million years younger than Weng’an. Using scanning electron microscopy, scientists uncovered exquisitely preserved microfossils, including spherical organisms and embryo-like structures containing internal cells.
None of the microfossils could be identified as animals, even though animal life was already established in other parts of the world by that point.
“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.”
Ross Anderson, an associate professor at Oxford University
Molecular Clocks Push Animal Origins Back by Millions of Years
Turning to molecular clock analyses, the research team calculated rates of evolution by comparing genetic differences among living species and working backward. When the team substituted older deposits — specifically the Bitter Springs Group in Australia, the Svanbergfjellet Formation in Norway, and the Chuar Group in Arizona, dating from roughly 850 to 730 million years ago — the estimated origin of crown-group animals shifted backward by roughly 200 million years.
Study senior author Ross Anderson noted that the findings open up the possibility that animals predated a series of extreme ice ages known as Snowball Earth
, which began around 720 million years ago and covered the planet’s surface in ice for extended periods.
Fossil Evidence For Pre-Ediacaran Animals Still Missing
Direct physical proof of pre-Ediacaran animals remains elusive. Orin Lole Durbin, a doctoral student at Virginia Tech and first author of the study, cautioned that the analysis does not prove that animals existed 800 million years ago, given the ongoing absence of body fossils from that interval.
The new Mongolian evidence undermines the reliance on younger calibrations, aligning more closely with previous chemical traces and lipid biomarkers suggesting that animals, including sponges, may have been present during the terminal stages of the Cryogenian period more than 635 million years ago, placing the earliest undisputed fossil evidence at roughly 574 million years ago during the Ediacaran period.