Scientists rebuilt ancient primate brains from fossils and found a stunning clue to human evolution: The brain may have grown larger not simply to think more, but because our ancestors were processing an increasing amount of visual information

For decades, scientists have faced a difficult question about primate evolution: why did the neocortex, the brain's outer layer involved in sensory perception, cognition and other complex functions, become disproportionately larger in primates than in other mammals?

The answer has been difficult to uncover because brain tissue does not fossilize.