In humans, the cerebral cortex, the tissuelike outermost layer of the brain, is thought to play a critical role in creative and analytical thinking.

But just how this portion of the brain evolved has remained a mystery.

Now researchers report that something resembling a cerebral cortex exists in the marine ragworm, a small creature with ancient roots that has not changed in hundreds of millions of years. The findings appear in the journal Cell.

“You can say that the topography is so similar that the human and worm must come from a common ancestor,” said Detlev Arendt, a researcher at the European Molecular Biology Laboratory and one of the study’s authors.