A new study suggests multicellular organisms existed more than 600 million years ago, about 60 million years before skeletal animals began appearing in a period known as the Cambrian Explosion. Back in 1998, scientists discovered strange fossils shaped as spheres in southern China. Since the discovery, researchers have been trying to figure out exactly what these fossils, known as Megasphaera, are.

Research, published this week in Nature, suggests that the fossils could be multicellular algae, or maybe even the embryos of ancient animals.

These Megasphaera were located in a rock layer in southern China known as the Doushantuo Formation. Each spherical fossil measures just 0.03 inches across and originates from a shallow marine environment.

The fossils are so old that no adult animal that might have produced these embryos has ever been found according to researchers.

“This opens up a new door for us to shine some light on the timing and evolutionary steps that were taken by multicellular organisms that would eventually go on to dominate the Earth in a very visible way,” said Shuhai Xiao, a professor of geobiology in the Virginia Tech College of Science, in a press release.

“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. This paper lets us put aside some of those interpretations.”

Speaking of light, that’s actually how the researchers observed the structure of the fossils. They sliced the rocks incredibly thin and shined a light through the fossils to see the structure, like a natural piece of stained glass.

Xiao and his team observed multiple cells bunched together. These cells varied in shape and size, and suggests the cells developed into different tissue types.

“While some hypotheses can now be discarded, several interpretations may still exist, including the multicellular fossils being transitional forms related to animals or multicellular algae. Xiao said future research will focus on a broader paleontological search to reconstruct the complete life cycle of the fossils,” a Virginia Tech press release reads.

The researchers published their findings in the journal Nature.

Image credit: Lei Chen and Shuhai Xiao

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