In 1758 the Swedish taxonomist Carolus Linnaeus dubbed our species Homo sapiens, Latin for “wise man.” It’s a matter of open debate whether we actually live up to that moniker. If Linnaeus had wanted to stand on more solid ground, he could have instead called us Homo megalencephalus: “man with a giant brain.”

Regardless of how wisely we may use our brains, there’s no disputing that they are extraordinarily big. The average human brain weighs in at about three pounds, or 1,350 grams. Our closest living relatives, the chimpanzees, have less than one-third as much brain—just 384 grams. And if you compare the relative size of brains to bodies, our brains are even more impressive.

As a general rule, mammal species with big bodies tend to have big brains. If you know the weight of a mammal’s body, you can make a fairly good guess about how large its brain will be. As far as scientists can tell, this rule derives from the fact that the more body there is, the more neurons needed to control it. But this body-to-brain rule isn’t perfect. Some species deviate a little from it. A few deviate a lot. We humans are particularly spectacular rule breakers. If we were an ordinary mammal species, our brains would be about one-sixth their actual size.


Competing theories seek to explain the value of a big brain. One idea, championed by psychologist Robin Dunbar of the University of Oxford, is that complicated social lives require big brains (pdf). A relatively large-brained baboon can make a dozen alliances while holding grudges against several rivals. Humans maintain far more, and more complicated, relationships.

Managing a social network can yield significant benefits: When a fight breaks out, it pays to have friends at your back. But keeping tabs on one’s social life requires effort. Dunbar and his colleagues have found that it takes longer for people to answer questions when they have to think about what’s going on in other people’s minds. And the more “mind reading” a question requires, the more it activates the brain.

A different perspective comes from Daniel Sol of the Center for Terrestrial Ecology and Applied Forestries in Barcelona, Spain, who studied the introduction of animals to new habitats. Comparing successful and unsuccessful invaders, he found that in both birds and mammals, big-brained species are more likely to be successful than are small-brained ones (pdf). This research suggests that larger brains are more adept at problem solving, which translates into a better chance of survival.

Twenty-five percent of all the calories you eat each day end up fueling the brain. For a newborn infant, with its little body and relatively large and fast-growing brain, that figure leaps to 87 percent.

It is possible that both Dunbar and Sol are at least partially correct. But no matter how the debate resolves, a puzzling question remains: If big brains are so useful, then why are they relatively rare? The answer is that nothing in nature comes for free—and where the brain is concerned, the cost can be enormous. In fact, scientists are discovering that a lot of human biology has reorganized itself to cope with the burden of an oversize brain.

in 1995 Leslie Aiello, then of 
University College London, and Peter Wheeler of Liverpool John Moores University offered the first possible reason for the rarity of big brains. Neurons, they pointed out, have a voracious appetite. They require lots of energy to produce their voltage spikes and to release neurotransmitters. They get that energy from oxygen and food, mostly glucose. A three-pound human brain burns up to 20 times as many calories as three pounds of muscle.

We cannot ignore this demand, even for a moment. A few minutes without oxygen may not do too much damage to our muscles but can irreparably harm the brain. The brain also requires a constant supply of food. Twenty-five percent of all the calories you eat each day end up fueling the brain. For a newborn infant, with its little body and relatively large and fast-growing brain, that figure leaps to 87 percent.

The brains of our ancestors more than 6 million years ago did not impose such brutal demands. Our forebears stood only about as tall as a chimpanzee and had brains the size of a chimp’s. For the next 4 million years, our ancestors remained small-brained. Then, around 1.8 million years ago, Homo erectus evolved. The first members of our genus that looked like us, 
H. erectus stood about as tall as modern humans, with brains that weighed around 900 grams. A half-million years ago, the brains of our ancestors started to grow again; 200,000 years ago they finally reached about the same weight as Homo sapiens brains today.