Australian research overturns the theory that the size of your brain determines intelligence.

Intelligence is more strongly associated with blood flow than the size of the brain, new research suggests

According to an Australian-led study, the human brain evolved to become not only larger but more "blood-thirsty" than previously believed.

Therefore, the more intelligent the more blood the brain required.

Scientists at the University of Adelaide calculated how blood flowing to the brain of our human ancestors changed over time by using the size of two holes at the base of the skull that allow arteries to pass to the brain.

It was found the size of the brain has increased by 350 per cent over human evolution, while blood flow to the brain increased an "amazing" 600 per cent, lead author Professor Emeritus Roger Seymour said.

"We believe this is possibly related to the brain's need to satisfy increasingly energetic connections between nerve cells that allowed the evolution of complex thinking and learning," he said.

The theory is that the more active the brain is, the more blood it requires.

"To allow our brain to be so intelligent, it must be constantly fed oxygen and nutrients from the blood," he said.

Co-author Dr Edward Snelling, from the University of the Witwatersand in South Africa, says the holes in the fossil skulls they studied accurately gauged the size of the arteries feeding the blood to the brain.

"The size of these holes show how blood flow increased from three million-year-old Australopithecus to modern humans. The intensity of brain activity was, before now, believed to have been taken to the grave with our ancestors," Dr Snelling said.

The study has been published in journal Royal Society Open Science.