Lei Chen and Shuhai Xiao. Spherical fossils like these may represent the earliest examples of what later became lineages of plants or algae

Animal diversity has led to an extraordinary array of body structures, but 600 million year old fossils suggest it may have all started from something resembling a cratered planet.

Modern Megasphaera are, as their name suggests, large spheres. They're a form of Firmicutes bacteria, although there is debate as to exactly which class they belong. When apparently similar fossils were found in 600 million year old rocks in southern China, they were thought to be early examples of the same thing, to the point of being given the same name. However, Virginia Tech geobiologist Professor Shuhai Xiao thinks they represent something far more significant.

The fossils are just 0.7mm across, tiny by animal standards, but large for bacteria. The Doushantuo Formation in which they are found is thought to have been a shallow sea at the time. In 1998, Xiao suggested the spheres were embryos of early animals, but with no signs of adult equivalents, he is now rethinking that position.

Instead Xiao has proposed in Nature that the spheres are actually adult animals, among the earliest in the animal kingdom. The idea has been proposed before, but Xiao and colleagues have strengthened the case with the discovery of more examples from different rock types in the same area. "Fossils similar to these have been interpreted as bacteria, single-cell eukaryotes, algae, and transitional forms related to modern animals such as sponges, sea anemones, or bilaterally symmetrical animals," Xiao said. "This paper lets us put aside some of those interpretations."

The first finds were made in gray rocks, but the Doushantuo Formation also contains black phosoporites, whose dark color make seeing fossils hard. Xiao's team sliced the black rocks so thinly they were able to see fossils others missed, which they claim “represent later developmental stages” of the spherical species.

An examination of the fossils' cells showed that they form spherical clusters and individual cells differ in shape and size. "That is a telling sign of the complexity of multicellular organisms that you don't find in bacteria or protists,” Xiao told LiveScience. He considers the cell differentiation evidence that assorted tissue types were forming.

Most intriguingly, the team found clusters of smaller cells dubbed “matryoshkas”, which may be reproductive cells. "This is a big thing, because if you look at modern multicellular organisms, including animals, this is a critical step towards very complex multicellular organisms,” Xiao said.

Xiao believes the spheres are an important stage in the transition from single-celled life to multicellular creatures, but admits their resemblance to some algae could indicate they are early plants, not animals.

"From the living animal point of view, we only have a certain morphology to go with. But there are extinct animals or even offshoots of the lineages leading to animals that could be rather different from what we know as animals living today." Xiao said. In his previous work, Xiao suggested the spheres might provide evidence that lineages of animal species diverged well before macroscopic evidence of their diversity of shapes becomes clear in the fossil record. He now thinks this gap could be by as much as 60 million years.