This week, a study in the same prestigious journal presents evidence for reinterpreting the 600 million-year-old fossils from the Precambrian era as giant bacteria.
The discovery "complicates our understanding of microfossils thought to be the oldest animals," said lead author Jake Bailey, a graduate student in earth sciences at the University of Southern California.
Bailey made his discovery by combining two separate findings about Thiomargarita, the world's largest known living bacterium.
In 2005, Thiomargarita discoverer Heide Schulz, from the University of Hannover in Germany, showed that the bacterium promotes deposition of a mineral known as phosphorite.
The fossils identified as eggs and embryos in 1998 came from southern China's Doushantuo Formation, which is rich in phosphorite.
The source for the rare mineral was unknown. Bailey wondered if an ancient relative of Thiomargarita might have been involved.
"The idea is that these bacteria were causing these phosphorite deposits to form," Bailey said.
Also in 2005, University of Georgia marine biologists Samantha Joye and Karen Kalanetra, who are co-authors on Bailey's study, found that Thiomargarita can multiply by reductive cell division, a process rare among bacteria but typical of animal embryos.
Bailey knew that the fossils had been identified as embryos in part because they showed evidence of reductive cell division. Then he thought again about the phosphorite deposits.
"When I put those two pieces together, I said ?perhaps they're not animal embryos at all."
Bailey and his co-authors compared the size and geometrical properties of the Doshuanto fossils and modern Thiomargarita bacteria ?they were nearly identical.
Source:University of Southern California