Anderson and his colleagues wondered whether other fossil sites from periods when animals definitely existed actually contained animal fossils. To find out, they studied more than 140 samples from the Kheseen Biota in Mongolia, a deposit more than 40 million years younger than the Weng'an Biota. Because it is younger, the researchers hypothesized that the rock should hold evidence of animal life.

Using electron scanning microscopy, the researchers saw microscopic fossils preserved in extraordinary detail, including tiny, spherical organisms and embryo-like structures containing internal cells. Yet, despite the quality of preservation, they could not identify any of the microfossils as animals. That result matters, because by the time the Kheseen Biota was deposited, animals were already present elsewhere on Earth.

"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

With no fossil evidence to settle the question, the researchers turned to another common method for dating ancient life: molecular clocks. These estimate when evolutionary lineages diverged by comparing genetic differences among living organisms and combining those differences with rates of genetic change. For example, if two bird species differ by five DNA changes, and one change takes about 1 million years to accumulate, scientists could estimate the two species diverged around 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 constrained by the younger Weng'an deposit, the estimated origin of animals was comparatively recent. But the researchers also ran the analysis using older geological constraints — rock formations in Norway, Australia and Arizona, deposited between roughly 850 million and 730 million years ago, that scientists have used before in the search for animals' early ancestors. That change shifted the timeline back by roughly 200 million years, to between about 800 million and 700 million years ago.

The researchers urge caution, though. "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 an 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"?

Other clues hint that animals may have a longer history than the fossil record shows. Chemical fossils, known as biomarkers, have been interpreted as evidence consistent with sponges living at least 650 million years ago — tens of millions of years before the oldest known animal fossils.

If animals did originate 800 million years ago, their emergence would 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 described as "Snowball Earth." Whether animals survived this uncertain time remains an open question.

The researchers plan to search additional Neoproterozoic rocks with different environments and preservation conditions to gather more evidence for their analysis.

"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."