The first animals may have evolved up to 200 million years earlier than standard fossil records indicate, according to a modeling study published October 2 in Science Advances. While undisputed animal body fossils date back roughly 574 million years to the beginning of the Ediacaran period, researchers using molecular clocks now estimate that the lineage split between 800 million and 700 million years ago, deep within the Neoproterozoic era.
Re-evaluating Molecular-Clock Calibrations in Mongolia and China
To test the reliability of evolutionary timelines, scientists examined the Kheseen Biota in Mongolia, a collection of phosphatized microfossils dating to roughly 550 to 526 million years ago. Using scanning electron microscopy, researchers analyzed more than 140 samples and identified exquisite microfossil species, including acritarchs—tiny spherical organisms with spines and branching projections—along with embryolike fossils containing internal cells. Despite this remarkable level of preservation, none of the microfossils could be confidently identified as animals, even though animals were already present in other parts of the world at that time.
This absence challenges the dependability of the Weng’an Biota in China, a roughly 590-million-year-old Ediacaran deposit that preserves microscopic organisms in high detail without containing definitive animal remains. For years, scientists treated Weng’an as an upper limit for animal origins, assuming that if animals existed then, they would appear in the fossil assemblage. “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 Dr. Ross Anderson, a researcher at Oxford University’s Museum of Natural History.
Connecting Neoproterozoic Rock Deposits to Earlier Evolutionary Timelines
Because the Kheseen Biota demonstrates that animals can be absent from exceptionally preserved sediments even when we know they existed elsewhere, the research team argues that rock conditions or environmental factors can easily obscure the earliest animal life. When the scientists substituted older, Tonian-age deposits—such as the Svanbergfjellet Formation in Norway, the Bitter Springs Group in Australia, and the Chuar Group in Arizona—into their molecular-clock analyses, the resulting divergence estimates for crown-group animals stretched back to roughly 800 to 700 million years ago.
This adjustment pushes potential animal origins before or during some of Earth’s most extreme ice ages. While chemical traces like lipid biomarkers in older rocks hint at sponges existing more than 635 million years ago during the Cryogenian period, definitive body fossils from that window remain undiscovered. Karma Nanglu, an assistant professor of evolutionary paleobiology at the University of California, Riverside who was not involved with the study, told Live Science that the research targets an evolutionary black box between actual origin and the first fossil window.
Frequently Asked Questions About Early Animal Evolution
What makes the Kheseen Biota different from older fossil sites?
The Kheseen Biota in Mongolia consists of microfossils dated to roughly 550–526 million years ago that show exceptional preservation comparable to China’s Weng’an Biota. Because animals were actively evolving globally during the Kheseen period yet still left no body fossils in the deposit, researchers proved that the absence of fossils does not mean animals were absent from Earth.
How do molecular clocks estimate animal origins?
Molecular clocks use genetic differences observed between living species to calculate roughly when their ancestral lineages split in the past. By recalibrating these models with older geological benchmarks instead of younger Ediacaran upper limits, researchers push the estimated birth date of the animal kingdom further back into the Neoproterozoic era.
Why haven’t scientists found body fossils from 800 million years ago?
Pre-Ediacaran animal body fossils continue to elude researchers because ancient environments may have lacked the specific chemical conditions required for fossilization, or scientists simply have not yet uncovered the right deposits.
The Search for Pre-Ediacaran Evidence
Orin M. Lole Durbin, a Virginia Tech paleontologist and author of the study, emphasized that the new molecular-clock modeling does not definitively prove animals swam in the oceans 800 million years ago. Instead, the findings remove a major methodological barrier that previously restricted scientific models to the Ediacaran interval. Researchers note that future work must explore fossil deposits from different parts of the world, representing various environmental conditions and modes of fossilization.
“Pre-Ediacaran animal body fossils still elude us, and this analysis does not prove that animals existed 800 million years ago,” Lole Durbin said.
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