Previously, the earliest fossil evidence of animals dated to around 574 million years ago, during the Ediacaran period (635 million to 538 million years ago). In the new paper, a research team argues that we can't just rely on the spotty fossil record, and suggests that animals first appeared up to 200 million years earlier than the oldest known animal fossils.
This would place the emergence of animals deep within the Neoproterozoic era (1 billion to 539 million years ago), possibly before or during some of Earth's most extreme ice ages.
"Until that evidence becomes available, the precise birth date 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.
"So the game is basically how much does it predate our first fossil discovery by and what happened in that evolutionary black box between when they actually originated versus when we get our first fossil window," Karma Nanglu, an assistant professor of evolutionary paleobiology at the University of California, Riverside who wasn't involved with the study, told Live Science.
To find out, Anderson and his colleagues studied more than 140 samples from the Kheseen Biota in Mongolia, which is more than 40 million years younger than the 590 million-year-old Weng'an Biota deposit. Given that it's a younger deposit, the researchers hypothesized that there should be evidence of animal life left in the rock. Using electron scanning microscopy, the researchers saw microscopic fossils preserved in extraordinary detail, including tiny, spherical organisms and embryo-like structures containing internal cells.
An electron microscope image of Asterocapsoides wenganensis, a microscopic fossil from the Kheseen Biota in Mongolia. (Image credit: Derek Briggs.)"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," Anderson said. "The Kheseen microfossils are just as well-preserved, yet animals continue to be absent — despite the fact we know at that point they existed."
Using molecular clocks
"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.
"Pre-Ediacaran animal body fossils still elude us, and this analysis does not prove that animals existed 800 million years ago," study first author Orin Lole Durbin, a doctoral student of paleobiology at Virginia Tech who was a University of Oxford undergraduate at the time of the study, said in the statement. "Meanwhile, our molecular-clock analyses show how much further back their evolutionary history could extend."
Could animals have lived before "Snowball Earth"?
Chemical fossils, or biomarkers, have been interpreted as evidence consistent with sponges living at least 650 million years ago, tens of millions of years prior to the oldest known animal fossils.
Related storiesIf animals did originate 800 million years ago, their emergence would also intersect with the Cryogenian period, which began around 720 million years ago as part of the Neoproterozoic era. During this interval, Earth experienced enormous glaciations often referred to as "Snowball Earth."
"Fossils themselves are by definition kind of hidden from general sight," Nanglu said."Beyond that there's an even more hidden world, and that has a lot of potential to tell us about animal origins."
Think you know about natural selection? Try your hand at our evolution quiz!
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