Brain’s blood appetite grew faster than its size

The brains of human ancestors didn’t just grow bigger over evolutionary time. They also amped up their metabolism, demanding more energy for a given volume, a new study suggests.

Those increased energy demands might reflect changes in brain structure and organization as cognitive abilities increased, says physiologist Roger Seymour of the University of Adelaide in Australia, a coauthor of the report, published online August 31 in Royal Society Open Science.

Blood vessels passing through bones leave behind holes in skulls; bigger holes correspond to bigger blood vessels. And since larger vessels carry more blood, scientists can use hole size to estimate blood flow in extinct hominids’ brains. Blood flow in turn indicates how much energy the brain consumed. (In modern humans, the brain eats up 20 to 25 percent of the energy the body generates when at rest.)

Seymour and colleagues focused on the carotid arteries, the vessels that deliver the bulk of the brain’s blood. The team looked at nearly three dozen skulls from 12 hominid species from the last 3 million years, including Australopithecus africanus, Homo neanderthalensis and Homo erectus. In each, the researchers compared the brain’s overall volume with the diameter of the carotid artery’s tiny entrance hole at the base of the skull.

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“We expected to find that the rate of blood flow was proportional to the brain size,” Seymour says. “But we found that wasn’t the case.” Instead, bigger brains required more blood flow per unit volume than smaller brains.

The boost in blood flow, and therefore metabolism, suggests two possible conclusions, Seymour says. As hominid brains got bigger, they might have packed in more nerve cells, or their nerve cells might have fired more frequently. Either way, he argues, the increased blood flow suggests greater brainpower, perhaps reflecting reorganization of the brain over the course of evolution.

But not all of the blood coming into the brain through the carotid arteries directly supports mental prowess. “You need more complicated wiring for bigger and more cognitively advanced brains,” says Dean Falk, an evolutionary anthropologist at Florida State University in Tallahassee. “But those brains have more advanced cooling requirements.”

Some of the blood coming in through the carotid arteries absorbs heat generated by the brain’s activity and then drains away, helping to keep the brain cool, Falk says. So while the study is a scientifically rigorous look at metabolism and blood flow toward the brain, she says, a follow-up study is needed to account for the blood moving away from the brain.