Early animals dethroned
Cell division in controversial fossils places them outside the animal kingdom
The oldest known “animal” fossils may have been living in the wrong kingdom. New images suggest that 570-million-year-old, many-celled blobs from China are not animal embryos as once thought, but rather some kind of spore-releasing cyst.
“They are not animals and they are not embryos,” says Stefan Bengtson, a paleobiologist at the Swedish Museum of Natural History in Stockholm. The work, done with colleagues including Philip Donoghue of the University of Bristol in England, appears in the Dec. 23 Science.
The fossils come from the Doushantuo rock formation in southern China. Once thought to represent algae, the fossils were reclassified as animals in 1998 based on how their cells divided. Blobs caught in various stages of division showed that they split into two cells, then four, then eight and so on without the organism growing larger — a process called palintomy.
Scientists have debated ever since what this cell division really means, with some arguing that the fossils represent giant bacteria rather than multicellular creatures.
Bengtson’s group used high-energy X-rays to probe 450 Doushantuo fossils. By tracing the blobs’ growth into more advanced developmental stages than other scientists had done, the team found that the fossils didn’t start to develop new tissue types the way animals would. Instead, the cells kept dividing through palintomy, creating more tiny cells that eventually lost their shape and got squeezed out as if being released as spores.
“I think it’s even more interesting than if they had been animals,” Bengtson says.
The scientists suspect the fossils may belong to a group of organisms known as Mesomycetozoea, which arose after the last common ancestor of animals and fungi but before the last common ancestor of living animals.
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The title of world’s oldest animal fossil remains unclaimed. In September in Evolution & Development, Bengtson and Chinese colleagues propose that another fossil, of around the same age as the Doushantuo blobs, is an early type of comb jelly. But that interpretation remains controversial, as do many other claims of early animals.
Finding the earliest animals may come down not to finding their physical remains but to spotting ways in which they changed their environment, writes Nicholas Butterfield, of the University of Cambridge in England, in a commentary accompanying the paper.