Thinking Power Grows Faster Than Brain Size


Stephen Luntz

Freelance Writer

clockAug 31 2016, 15:19 UTC
Hominin skulls

The difference in skull volumes in our evolution may be less important for measuring intelligence than the diameter of small holes at the bottom. R.S. Seymour, V. Bosiocic, and E.P. Snelling

The evolution of humanity is written in fossils revealing our expanding brains, but the relationship between brain size and thinking capacity may not be linear. Instead, a new study suggests that as the brain got larger our computing power grew faster still. Humans needed to increase their blood supply to feed the increasingly hungry beast at the top of our spines.

Just as we don’t have intact brains from humanity’s ancestors, we have no direct measure of how much blood their brains were supplied. Nevertheless, the fossil record provides us with proxies. The volume of our ancestors’ skulls points to the size of the brain within, while the diameter of holes at the base of the skull indicates the size of the arteries that fed them.


Professor Emeritus Roger Seymour of the University of Adelaide measured the size of these holes and estimated how the blood flow for Homo sapiens compares with 12 hominid species going back to Australopithecus afarensis three million years ago.

“Brain size has increased about 350 percent over human evolution, but we found that blood flow to the brain increased an amazing 600 percent,” Seymour said in a statement. “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.”

In Royal Society Open Science, Seymour reports an exponential relationship between brain volume and blood flow. A doubling in brain size is associated with an increase of about 2.6 times in blood flow.


Growth in blood flow over the last three million years. R.S. Seymour, V. Bosiocic and E.P. Snelling/Open Science

Seymour told IFLScience blood flow is a reflection of the nutrition the brain requires, and therefore its metabolic rate. “Previously it was assumed that intelligence and brain metabolism are directly related to brain size,” he said. “But we found the metabolic rate increased faster." Although the number of nerve cells is proportional to volume, but the number of connections between cells increases more rapidly than the number of cells themselves. 

Seymour thinks it is the number of connections that determine intelligence, likening the connections to the computing power of a supercomputer. He notes that the power such a machine draws is proportional to the computations it performs, and sees the blood supply as analogous to electricity.


Co-author Vanya Bosiocic added in a statement: “Throughout evolution, the advance in our brain function 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.” However, Seymour told IFLScience the fossil record provides insufficient detail to reveal if our evolution involved any sudden jumps in processing power, for example, associated with the domestication of fire.

Considering how important it is for our brains to be fed with sugar and oxygen, we know surprisingly little about how this works. It is only this month that scientists reported the discovery of a process by which the brain takes up sugar from the blood, rather than passively receiving it, as previously suspected.

These holes are easy to overlook, but their size determines how fast we can think. R.S. Seymour, V. Bosiocic and E.P. Snelling/Open Science

  • tag
  • human evolution,

  • brain capacity,

  • blood flow