The specimen, about .7 mm across, is tricky to categorize. (Lei Chen and Shuhai Xiao)

In 1998, scientists found some strange fossils in southern China: The spherical clusters of cells, 600 million years old, seemed to be unlike anything alive today.

Since then, researchers have posed different theories on what sort of species this Megasphaera might have been — but many thought that the fossils showed an embryo of some early animal, or clusters of single-celled organisms like bacteria. New research published in Nature reviews new specimens of the same organism, and suggests that they might actually be early-stage animals or algae — multi-celled organisms that existed so early in our evolutionary past that no living descendants remain close enough for us to compare them to.

The smaller structure growing inside the fossil might indicate multicellularity. (Lei Chen and Shuhai Xiao)

Corresponding author Shuhai Xiao, professor of geobiology at Virginia Tech, said that he and his colleagues found evidence that the fossils contain more than one type of cell. "When you have a ball of cells, it keeps dividing and dividing to form more, but the ball doesn't grow — the cells get smaller and smaller," Xiao said. "In a multicellular organism, some types of cells will start to divide more rapidly at the expense of others. If some kinds of cells are growing much faster, others have to die."

Because Xiao and his colleagues saw evidence of this cell death, they believe the organisms must be multicellular —like plants and animals — even though they were much simpler than anything that lives today.

Whatever they were, the organisms lived in what was probably a very shallow, warm marine environment, not unlike our modern-day Bahamas, Xiao said.

"How these actually fit into the family tree of animals and other multicelled organisms is still a big question," Xiao said. "The bottom line is that we think we're looking at some of the early evolutionary steps towards complex multicellularity."

It's possible that these fossils show an early step of the development of algae, not animals. "It's very different from the stereotypical animal development, so it's hard to say," Xiao said. "They look alien to us."