Mongolian microfossils undercut the 590-million-year ceiling on the age of animals

Weng'an's role as a ceiling rests on one premise: rocks that keep microscopic organisms in such detail would have kept animals too, had any been alive ^[2]. The premise can be checked against a deposit from a time when animals certainly existed. Kheseen formed after about 550 million years ago ^[2], later than the oldest animal fossils at roughly 574 million years ^[13]. If a site that good can miss animals known to be alive, an equally good empty site 590 million years ago says little about whether animals were around.

"The Kheseen microfossils are just as well preserved, yet animals continue to be absent," said Ross Anderson, the senior author and an associate professor at Oxford's Museum of Natural History ^[7]. "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," he said ^[6]. The team's explanations are that animals lived in other environments, or that the local chemistry did not preserve them ^[8]. Those early animals would have been small and soft-bodied, according to the Oxford account ^[17].

The molecular clock is the next step. It compares genetic differences among living species and works backward, using fossil dates as reference points along with rates of evolution ^[10]. "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 was not involved, told Live Science ^[16]. With the older deposits as maximum ages, the estimate moved back about 200 million years from Weng'an-calibrated runs, to 800 to 700 million years ago ^[11]. Against the oldest animal fossils, the gap is roughly 126 to 226 million years ^[1].

The thing this doesn't tell you is how much of the new date comes from the new ceiling. Given maximum ages of 850 to 730 million years ^[9], the clock returned 800 to 700 million years, just under those bounds ^[11]. The older ceilings are also arguments from absence: fossil-rich deposits with the potential to preserve animals, where none have turned up ^[9]. The published accounts do not explain why that reasoning holds at 850 million years when Kheseen undercuts it at 590.

I think the negative result is the firmer finding: a well-preserved deposit without animals is weak evidence that animals were absent, while the 800-million-year date depends on the bounds chosen. Biomarkers consistent with sponges at least 650 million years ago ^[12] reach about 76 million years past the oldest animal fossils ^[3], well short of 800 million. "Until that evidence becomes available, the precise birth date of the animal kingdom remains uncertain," Anderson said in a statement ^[15].