First animals on Earth may have emerged between 800 and 700 million years ago. Publishing in Science Advances, researchers led by Orin Lole Durbin re-examined molecular-clock calibrations by studying fossil assemblages in Mongolia, challenging the assumption that the absence of animal fossils in certain Neoproterozoic rocks proves animals had not yet evolved.
The Neoproterozoic Fossil Record and the Sponge Biomarker Paradox
The oldest undisputed animal fossils, belonging to the Ediacaran biota, date back approximately 574 million years near the beginning of the Ediacaran period. However, lipid biomarkers—chemical traces specific to sponges—hint that animals were around before 635 million years ago. These chemical signatures suggest that animals were present during the terminal 10 to 20 million years of the Cryogenian period.
This creates a stark dichotomy in early evolutionary timelines. While molecular clocks use genetic differences between living species to estimate when lineages split, paleontologists rely on physical fossil evidence to calibrate those clocks. For years, exceptionally well-preserved microfossil deposits have been used to set maximum age constraints on when animal life could have begun.
How the Kheseen Biota Upended Established Maximum Age Calibrations
To test the reliability of one commonly used molecular-clock calibration, the research team examined the Kheseen Biota in Mongolia. This assemblage consists of phosphatized microfossils dating from roughly 550 to 526 million years ago. This specific timeframe occurred when animals were already present and evolving elsewhere on Earth, yet the Kheseen sediments contain no fossils that can be confidently identified as animals.
The Kheseen microfossils share several fossil species and possess a level of preservation comparable to China’s Weng’an Biota. The Weng’an Biota has been used as a maximum calibration for the origin of 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 Dr. Ross Anderson, a researcher at the Oxford University’s Museum of Natural History.
Given that animal evolution was already underway at the time the Kheseen layers formed, we cannot assume that their missing presence in the assemblage proves animals had not yet come into being. As a result, this discovery casts doubt on the validity of utilizing the Weng’an Biota as an upper calibration limit for the emergence of animals.
Recalibrating Molecular Clocks and the 800-Million-Year Horizon
The researchers substituted older, Tonian-age deposits into their molecular-clock analyses. The resulting data produced much older divergence estimates for the emergence of animals. Roughly 800 to 700 million years ago marks the timeframe that these estimates for crown-group animal origins stretched into, based upon the chosen calibration.
“Pre-Ediacaran animal body fossils still elude us, and this analysis does not prove that animals existed 800 million years ago,” Orin Lole Durbin noted. “However, our new fossil evidence from Mongolia undermines one of the main arguments for restricting animal origins to the Ediacaran interval.” “Meanwhile, our molecular-clock analyses show how much further back their evolutionary history could extend.”
Researchers should investigate fossil strata across diverse global regions in upcoming studies, encompassing varied environments and fossilization processes, to hunt for ancient animals. All available clues regarding animal life ought to be factored into this pursuit, spanning body fossils, trace fossils, and chemical biomarkers.
Until that evidence becomes available, the precise birth date of the animal kingdom remains uncertain.
