Human intelligence can no longer be explained as just the evolutionary increase in the size of the brain's frontal lobes, British researchers say.

A study by Durham and Reading universities into the comparative size of the frontal lobes in humans and other species has determined that they aren't, as previously believed, disproportionately enlarged relative to other areas of the brain, a Durham release reported Wednesday.

Therefore the size of frontal lobes cannot solely account for humans' superior cognitive abilities, the researchers said; supposedly more "primitive" areas, such as the cerebellum, were equally important in the expansion of the human brain.

These areas may therefore play previously unsuspected roles in human cognition and its disorders such as autism and dyslexia, they said.

"Probably the most widespread assumption about how the human brain evolved is that size increase was concentrated in the frontal lobes," Durham anthropologist Robert Baron said.

"It has been thought that frontal lobe expansion was particularly crucial to the development of modern human behavior, thought and language and that it is our bulging frontal lobes that truly make us human," he said.

"We show that this is untrue: human frontal lobes are exactly the size expected for a non-human brain scaled up to human size."

Many high-level cognitive abilities are carried out by more extensive networks linking many different areas of the brain, the researchers argue, suggesting the structure of these networks -- more than the size of any particular brain region -- is critical for cognitive functioning.