A 520-million-year-old, three-inch (7.6-centimetre) fossil has yielded evidence that complex brains evolved much earlier than previously thought, scientists said Wednesday.

The preserved external skeleton of Fuxianhuia protensa, an extinct type of arthropod, is the earliest known fossil to show a complex brain, according to a study published in the journal Nature.

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“No one expected such an advanced brain would have evolved so early in the history of multicellular animals,” co-author Nicholas Strausfeld, a neurobiologist at the University of Arizona, said in a statement.

The fossil was deposited in mudstone during the Cambrian period in what is today China’s southwestern Yunnan Province.

It was a member of the family of arthropods, creatures without backbones which today include insects, spiders and crustaceans.

The researchers found two eyes on stalks which contained traces of a substance they interpreted to be nerve tissue — optic nerves connected to a three-segment brain.

“This fossil provides the most convincing, and certainly the oldest, description of nervous-system tissue in a fossil anthropod,” Graham Budd of Sweden’s Uppsala University Earth Sciences Department wrote in a comment on the study.

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He pointed out that soft tissue like brain matter is much less likely to be preserved in the fossil record than bone and shell as it decayed much more easily.

The team also claimed that their findings settled a long-standing scientific argument about the evolution of insects.

They said their research ruled out branchiopods, shellfish with much simpler brains, as direct ancestors of insects — lumping today’s bugs instead with another arthropod line that includes crabs and shrimp.

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“In principle, Fuxianhuia’s is a very modern brain in an ancient animal,” added Strausfeld.

“It is remarkable how constant the ground pattern of the nervous system has remained for probably more than 550 million years.”