The deep folds that give the adult human brain its wrinkled walnut appearance were Nature’s solution to fitting a large, powerful processor into a small skull.

Like a piece of flat, square paper crumpled together to fit into a small, round hole, folding allows more neurons to be packed closer together, with shorter, faster connections between them.

While scientists have long understood why there are folds in the brain’s outer layer, called the cerebral cortex or grey matter, the how has remained a mystery.

Do the creases develop as a result of genetic, biological or chemical signals? Or are they caused by physical forces?

On Monday, a team of researchers from the United States and Europe said the folds can be explained by physics — a discovery that may have important implications for understanding certain brain disorders.

Folds in the cortex develop through buckling in weak spots which develop as the foetal brain grows, they said.

The brains of human foetuses are smooth for about the first 20 weeks, when folding begins and continues until the child is about 18 months old.

The surface area covered by the folded cortex is almost three times that of a smooth brain the size of our head, study co-author Lakshminarayanan Mahadevan from Harvard University in Massachusetts told AFP.

“The number, size, shape and position of neuronal cells during brain growth all lead to the expansion of the gray matter, known as the cortex, relative to the underlying white matter,” he said by email.