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While it is tough to work out just how our brains got so big, one thing is certain: all that thinking requires extra energy. The brain uses about 20 per cent of our energy at rest, compared with about 8 per cent for other primates. “It’s a very metabolically demanding tissue,” says , an evolutionary biologist at Duke University in Durham, North Carolina.

In the past year, three mutations have been discovered that may have helped meet that demand. One emerged with the publication of the gorilla genome, in March (). This revealed a DNA region that underwent accelerated evolution in an ancient primate ancestor, common to humans, chimps and gorillas, some time between 15 and 10 million years ago.

The region was within a gene called RNF213, the site of a mutation that causes Moyamoya disease – a condition that involves narrowing of the arteries to the brain. That suggests the gene may have played a role in boosting the brain’s blood supply during our evolution. “We know that damaging the gene can affect blood flow, so we can speculate that other changes might influence that in a beneficial way,” says , an evolutionary geneticist at the Sanger Institute in Cambridge, UK, who was part of the group that sequenced the gorilla genome.

There are more ways to boost the brain’s energy supply than just replumbing its blood vessels, though. The organ’s main food source is glucose and this is drawn into the brain by a glucose-transporter-molecule in the blood vessel walls.

Compared with chimpanzees, orang-utans and macaques, humans have slightly different “on switches” for two genes that encode the glucose transporters for brain and muscle, respectively (). The mutations mean more glucose transporters in our brain capillaries and less in our muscle capillaries.

“It’s throwing a switch so you divert a greater fraction [of the available glucose] into the brain,” says Wray. In short, it looks like athleticism has been sacrificed for intelligence.

“The mutation threw a switch, diverting a greater fraction of glucose into the brain. It looks like athleticism was sacrificed for intelligence”