Human intelligence evolved thanks to abnormally fast blood flow to the brain

An ever-increasing and disproportionately high blood flow to the brain over three million years of human evolution may be the reason we became so intelligent. By working out the rate of cerebral blood flow in 12 species of our hominin ancestors, scientists have shown how it could have led to better cognitive ability over time.

Roger Seymour, from the University of Adelaide, and colleagues, measured the size of the internal carotid arteries – which pumps blood into the brain – to see how it changed in early human species over time – from Australopithecus to the first Homo sapiens. Their findings, published in the Royal Society journal Open Science, showed that while brain size increased 3.5 fold over three million years, cerebral blood flow increased six fold.

Our large brains are largely what distinguishes us from other primates and is the reason we became intelligent as a species. To understand how our brains got so big, scientists have been looking to understand the organ's metabolic evolution. This is related to how much energy the body allocates to brain function. In humans, this is between 20 and 25%, while other primates dedicate between 8 and 10%.

It has been proposed that the cerebral metabolic evolution could have led to increased intelligence by establishing the communication pathways needed for higher cognitive ability.

Scientists measured the dimensions of the carotid foramen (which allows passage of the internal carotid arteries into the skull) in 12 different hominin species. In total, 35 specimens were measured. Their findings showed a vastly increased rate of blood flow in comparison to the increase in brain size over the three million years of evolution.

"The evolution of human cognition has been inferred from anthropological discoveries and estimates of brain size from fossil skulls," the researchers wrote. "During [three million years] of hominin evolution, cerebral tissue perfusion increased 1.7-fold, which, when multiplied by a 3.5-fold increase in brain size, indicates a six-fold increase in total cerebral blood flow rate. This is probably associated with increased interneuron connectivity, synaptic activity and cognitive function, which all ultimately depend on cerebral metabolic rate."

Continuing, they said they believe the disproportionate increase in blood flow points to an evolutionary reorganisation of the brain. They said cerebral blood flow would have allowed for "an increase in the number of synapses per neuron or greater synaptic activity" and that this "would have facilitated enhanced information processing and communication pathways as cortical enlargement and regional specialisation took place".