How the human brain became bloodthirsty

A study led by the University of Adelaide has found that the evolution of the human brain is closely linked with an increase in its blood supply. This is in contrast to previous anthropological discoveries, which have claimed that our increasing cognition was a result of the growing size of the brain (as indicated by skull size).

The study saw honours student and study co-author Vanya Bosiocic travel to South Africa and work with world-renowned anthropologists on the oldest hominin skull collection, including the newly discovered Homo naledi. There, she and her colleagues calculated how blood flowing to the brain of human ancestors changed over time, using the size of two holes at the base of the skull that allow arteries to pass to the brain.

“Ancient fossil skulls from Africa reveal holes where the arteries supplying the brain passed through,” said co-author Dr Edward Snelling, from South Africa’s University of the Witwatersrand. “The size of these holes shows how blood flow increased from three million-year-old Australopithecus to modern humans.”

“Brain size has increased about 350% over human evolution, but we found that blood flow to the brain increased an amazing 600%,” added project leader Professor Roger Seymour, from the University of Adelaide. “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.”

Professor Seymour explained that intelligent brains must be constantly fed oxygen and nutrients from the blood. He said, “The more metabolically active the brain is, the more blood it requires, so the supply arteries are larger. The holes in fossil skulls are accurate gauges of arterial size.”

So what caused this evolution in brain function in the first place? According to Bosiocic, it “appears to be related to the longer time it takes for us to grow out of childhood. It is also connected to family cooperation in hunting, defending territory and looking after our young.

“The emergence of these traits seems to nicely follow the increase in the brain’s need for blood and energy,” she said.

The study has been published in the journal Open Science.

Image caption: Internal carotid foramina in selected hominin species: (a) Australopithecus africanus, (b) Homo neanderthalensis (cast) and (c) archaic Homo sapiens. All photographs are the same scale (increments in b and c are 0.5 mm) and thus illustrate the increase in foramen size across hominin evolution.