The first animals may have appeared on Earth hundreds of millions of years earlier than their fossils suggest, a new modeling study suggests. Previously, the oldest fossil evidence of animals dated back to about 574 million years ago, during the Ediacara period (635 million to 538 million years ago). In the new paper, a research team argues that we cannot simply rely on the patchy fossil record and suggests that animals first appeared up to 200 million years before the oldest known animal fossils. In the study, published Friday (Oct. 2) in the journal Science Advances, the researchers argue that the absence of animal fossils in some exceptionally well-preserved ancient rocks does not mean that animals were absent from history. Instead, they suggest that the fossil record may be missing a key chapter of animal evolution, which could push the origins of animals back to between 800 and 700 million years ago. It doesn’t mean that animals didn’t live at that time. Fossils may be missing from rocks for a variety of reasons, including because scientists are looking in the wrong places or because conditions were not suitable for creating fossils, making it difficult to determine the first evolved animals. “Until that evidence is available, the precise date of birth of the animal kingdom remains uncertain,” study co-author Ross Anderson, an associate professor of natural history at the University of Oxford, said in a statement. Where are the animals? Until now, most researchers relied on well-preserved rocks from the Ediacaran period, including the approximately 590 million-year-old Weng’an Biota in China, to investigate the timing of animal evolution. Because rock deposits contain uniquely preserved microscopic organisms, but not definitive animal fossils, experts have sometimes treated them as a kind of upper limit for when animals might have evolved. Get the world’s most fascinating discoveries delivered straight to your inbox. “So the game is basically how much predates our first discovery of fossils and what happened in that evolutionary black box between the time they actually originated and the time we got our first window of fossils,” said Karma Nanglu, assistant professor of evolutionary paleobiology at the University of California, Riverside. But researchers wondered whether other fossil sites from known animal periods actually had evidence of animal fossils. To find out, Anderson and his colleagues studied more than 140 samples of the Kheseen Biota in Mongolia, which is more than 40 million years younger than the 590-million-year-old Weng’an Biota deposit. Since it is a younger deposit, the researchers hypothesized that evidence of animal life should remain in the rock. Using scanning electron microscopy, researchers saw microscopic fossils preserved in extraordinary detail, including tiny, spherical organisms and embryo-like structures containing internal cells. What to read next An electron microscope image of Asterocapsoides wenganensis, a microscopic fossil from the Kheseen biota in Mongolia. (Image credit: Derek Briggs.) However, despite the quality of the preservation, the researchers were unable to identify any of the microfossils as animals. This is important, because when the Kheseen Biota was deposited, animals were already present elsewhere on Earth. “The Kheseen Biota breaks the argument that the exceptional microfossils from Weng’an mean we would have seen animal fossils in the assemblage if they had existed at the time,” Anderson said. “The Kheseen microfossils are just as well preserved, but the animals are still absent, even though we know they existed at the time.” Using Molecular Clocks Given the absence of fossil evidence, Anderson and his collaborators turned to another common method used to date ancient life: molecular clocks. Molecular clocks estimate when evolutionary lineages diverged by comparing genetic differences between living organisms and combining those differences with rates of genetic change. For example, if two species of birds have five DNA changes and one DNA change takes about 1 million years to accumulate, scientists could estimate that the two species diverged about 5 million years ago. “Molecular clocks are useful when we don’t have a particularly good fossil record for a group of interest or a time period of interest,” Nanglu said. When the analysis was limited by the younger Weng’an deposit, the estimated origin of the animals was comparatively recent. When they used older geological constraints, specifically rock formations in Norway, Australia and Arizona deposited between 850 and 730 million years ago that have been used in the past to try to find the first ancestors of animals, the timeline changed. By comparing the DNA of modern animals with reference dates from the fossil record along with rates of evolutionary change, their estimate went back about 200 million years, to about 800 million and 700 million years ago. “Fossils of pre-Ediacaran animal bodies still elude us, and this analysis does not prove that animals existed 800 million years ago,” said the study’s first author, Orin Lole Durbin, a doctoral student in paleobiology at Virginia Tech, who at the time was a student at the University of Oxford. of the study, he said in the statement. “Meanwhile, our analyzes of the molecular clock show how far back its evolutionary history could go.” Could animals have lived before “Snowball Earth”? There are other clues that the animals may have a longer history. The chemical fossils, or biomarkers, have been interpreted as evidence consistent with sponges living at least 650 million years ago, tens of millions of years earlier than the oldest known animal fossils. If animals originated 800 million years ago, their emergence would also intersect with the cryogenic period, which began around 720 million years ago as part of the Neoproterozoic era. During this interval, the Earth experienced enormous ice ages often referred to as “Snowball Earth.” Although there is no definitive answer as to whether the animals survived this uncertain time, the researchers plan to search for additional Neoproterozoic rocks with different environments and preservation conditions to find more evidence for their analysis. Lole Durbin, Orin M. Anttila, Elie SC Briggs, Derek EG Macdonald, Francis A. Anderson, Ross P. (202) Reassessing molecular clock peak age calibrations revives pre-Ediacara divergence estimates for animals. Scientific advances. 12. 10.1126/sciadv.aeg6289 Do you think you know natural selection? Try your hand at our evolution quiz!
