“What would you do with a brain if you had one?” Dorothy’s question to the Scarecrow in The Wizard of Oz elicited one of the movie’s most delightful songs, in which her straw-filled friend assured her that, among other things, he could “think of things I’d never thunk before.” But the Scarecrow seemed to do quite well without one, thus avoiding the high energy costs of fueling and cooling a human brain—which, with an average volume of about 1,400 cubic centimeters, is humongous relative to our body size.
How did our brains get so big? Researchers have put forward a number of possible explanations over the years, but the one with the most staying power is an idea known as the social brain hypothesis. Its chief proponent, psychologist Robin Dunbar of Oxford University, has argued for the past two decades that the evolution of the human brain was driven by our increasingly complex social relationships. We required greater neural processing power so that we could keep track of who was doing what to whom.
Our expanded brains could have been practical for other things, of course, such as innovations in tool use and food gathering. Most researchers, including Dunbar, agree that these hypotheses are not mutually exclusive. Whatever the reasons for the very large human noggin, there is a lot of explaining to do, because big brains have a lot going against them.
The oversized Homo sapiens brain let us take over the planet, build cities, send space probes to Mars, and do all the other marvelous things that we humans are so proud of. But none of these things makes us much better at reproducing, and in terms of evolution, that’s really all that matters. It’s not so obvious why Darwinian natural selection should have favored the brain’s dramatic expansion given the huge costs. Although the human brain is only about 2 percent of total body weight, it siphons off about 20 percent of our total calorie intake; this overall percentage varies little whether we are engaged in hard mental tasks or just zoning out.
If having a large brain were all that advantageous, it seems that every animal would have one. And yet most species have been content, evolutionarily speaking, with relatively small ones. The brain of our closest living cousin, the chimpanzee, is less than a third the volume of ours, even though chimps weigh almost as much as humans.
Our primate lineage had a head start in evolving large brains, however, because most primates have brains that are larger than expected for their body size. The Encephalization Quotient is a measure of brain size relative to body size. The cat has an EQ of about 1, which is what is expected for its body size, while chimps have an EQ of 2.5 and humans nearly 7.5. Dolphins, no slouches when it comes to cognitive powers and complex social groups, have an EQ of more than 5, but rats and rabbits are way down on the scale at below 0.4.
Natural selection already had something to work with, if Dunbar’s hypothesis is correct, because many primates have complex social relationships. The best proxy for this social complexity, Dunbar argues, is the size of an animal’s social group. The social brain hypothesis really got cooking back in 1993, when Dunbar, along with anthropologist Leslie Aiello, crunched a bunch of numbers from the hominid fossil record and observations of living apes. The larger a species’ group size, they found, the larger its brain—particularly the neocortex, the outer layers where most of the serious thinking goes on. They concluded that the correlation among group size, brain size, and neocortex size held pretty tightly throughout our lineage, from the australopithecines more than 3 million years ago to modern-day humans.
The average group size for today’s Homo sapiens, the pair concluded in a widely cited paper in Current Anthropology, is about 150 people—a figure often called Dunbar’s number, and which refers to the number of people with whom the average person can maintain close personal relationships. (The Australopithecus afarensis Lucy had a group size of 60 to 70, as do most living apes such as chimps, but Neanderthals were a close social-group match with modern humans.)