When graduate student Jake Bailey hypothesized that one of the most significant fossil finds of the past decade might have been misinterpreted, it made him “a little nervous”, he says. But Bailey, with the support of his adviser Frank Corsetti, an Earth scientist at the University of Southern California in Los Angeles, pressed ahead.

As a student, Bailey followed several palaeontological controversies, including the 1998 discovery of 600-million-year-old microfossils in south China. At first, researchers thought they were algae, but others suggested, on the basis of their shape, size, structure and evidence of reductive cell division (the cleavage of a cell into a number of smaller cells, as occurs in the fertilized eggs of animals), that the 'Doushantuo microfossils' were tiny animal eggs and embryos — the world's oldest animal fossils. If correct, that assessment would have profound implications in helping scientists understand the evolution of cell division in early multicellular organisms. “But I never really thought seriously about it or about working on it myself,” Bailey says.

That changed when he read two articles from marine scientists who study similar-looking modern bacterial cells — giant sulphur bacteria. In one, Samantha Joye and Karen Kalanetra at the University of Georgia presented evidence of reductive cell division in these bacteria from the Gulf of Mexico. Bailey noticed that the shapes and structures of these modern organisms were very similar to the descriptions of the putative Doushantuo eggs. His interest was further piqued by evidence from Heide Schulz at the University of Hannover in Germany of biochemical activities in giant sulphur bacteria that promote the formation of phosphorite in their environment — the same type of mineral deposit that preserves the Doushantuo microfossils. The presence of phosphorites has puzzled scientists who believe that the Doushantuo fossils are animal eggs; the mechanism of the mineral's formation remains poorly understood and there's no evidence that animals contribute to the process.

After reading these two papers, “the possibility had to be considered”, says Bailey, that the Doushantuo microfossils might not be of animal eggs but of giant sulphur bacteria similar to those living in the Gulf of Mexico. He consulted Corsetti, who encouraged him to contact Joye and Kalanetra.

They agreed to share their samples and assist with the analysis. It only took a few weeks of imaging to confirm his hunch. “Right from the get-go there was this eureka moment,” Bailey says. On page 198, the team lays out its evidence that the Doushantuo microfossil 'embryos' are indeed fossilized giant sulphur bacteria.

Bailey admits to some trepidation about going against the conventional wisdom at this stage in his career. “It's intimidating to provide an alternative interpretation to something that some of the biggest names in the field have published earlier,” he says. But he also knows that scientific knowledge is a moving target, not a fixed destination. “There's still a lot to do, and I think the controversy might remain for many years,” Bailey says.