Scientists think they now know why all people have big brains and little jaws.
They say our heads look the way they do today because one of our genes - which determine things like your hair colour and height - changed during evolution.
They say early primate skulls - which look like today's gorillas and chimpanzees - had been "locked" by big muscley jaws that stopped skull growth.
But when the gene mutated, our jaws got weaker and our skulls could increase along with our brain size, they say.
Today's humans have massive brains compared to all other mammals.
Fossils of prehistoric humans feature rounder skulls, flatter faces, smaller teeth and weaker jaws.
They show how we have dramatically changed over the ages.
But they are all examples of the post-gene mutation, as it is suggested by the scientists.