The genetics behind the development of folds in the human brain have been a mystery, but there are new clues in a study scheduled for presentation Sunday at the annual meeting of the Society for Neuroscience, in San Diego.

As the human brain evolved, there was a dramatic increase in the number of brain cells and connections, the researchers said. But this growth was restricted by the size of the skull in relation to the birth canal, leading the brain to fold into ridges and valleys.

“The evolution of the human brain over time is a very complex process,” Elizabeth Atkinson, of Washington University in St. Louis, said in a Society for Neuroscience news release. “Our study connects the folding of the brain with the underlying genetics, and provides unique insight into how the evolution of our genes has driven the shape, and ultimately the function, of our brains.”