600-million-year old algae may represent the world’s oldest fossil and could shed light on the marine life during the interval in which molecular clocks predicting animal groups were still evolving.

A team, led by researchers from the University of Bristol in the U.K., has discovered that ancient fossils discovered in South China that are considered to be some of the earliest examples of animal remains could in fact be algae.

“Dated at around 600 million years old, these rocks preserve an assemblage of microscopic fossils, perfectly-aged to be candidates for the oldest evidence of animal life,” John Cunningham, Ph.D., from the University of Bristol’s School of Earth Sciences, said in a statement. “These fossils aren’t recognizable as remains of fully grown animals but some resemble embryos, ranging from single cells to clusters of thousands.

“The preservation is so exquisite, that even sub-cellular structures can be identified, including possible nuclei,” he added.

After reviewing the Weng’an Biota fossils, researchers revealed that none of the characteristics previously used to define the fossils as animals are actually unique to animals alone, which opens up alternative possibilities.

“Many proponents of animal affinity have argued that the Y-shaped junctions between the cells in the fossils are an important animal character, but this a feature common to many multicellular groups, including algae, that are very distant relatives of animals,” professor Phillip Donoghue, a co-author from the University of Bristol, said in a statement.     

Kelly Vargas, Ph.D., a postdoctoral researcher from the University of Bristol and one of the paper’s co-authors, added that there are some hints as to what the fossil’s identity is.

“But with the lack of adult forms that could indicate their identity, paleontologists have to rely on information from cellular anatomy to determine whether these tiny fossils belong to animals or to a different group,” Vargas said.