Why do we have big brains?
A simple answer to that question is: Because we’re primates, and primates are very brainy. Then the question becomes: Why do primates have big brains?
Many explanations for the evolution of primate intelligence relate to the challenges of finding food. Monkeys and apes need big brains to keep track of widely distributed, patchy and unpredictable foods like fruit. Or maybe they need enhanced intelligence to extract food embedded in a tough shell or to collect termites hiding in a mound.
Critics of such arguments have pointed out that these problems are not necessarily unique to primates. As an alternative, in the late 1980s, scientists suggested primates have big brains because they are highly social animals. Primates are not the only mammals that live in large groups, but monkeys and apes stand out, in general, for having very intense social relationships. In fact, watching a group of monkeys is kind of like watching a soap opera: Individuals have friends, but they also have enemies. They team up to form coalitions to overthrow their foes, but they also reconcile after a fight. They yield to the leaders of their group, but they also sneak off to engage in clandestine affairs when no one’s looking.
If you’re going to be involved in all of these social maneuverings, you need to be able to keep track of all sorts of social information—how you relate to others in the group, how third parties relate to one another—but more importantly, you need to be able to use that information to your benefit. And to do that, you need a big brain. That’s the basis of the Social Brain Hypothesis (PDF).
The biggest proponent of this hypothesis is Robin Dunbar, a professor of evolutionary anthropology at the University of Oxford. Dunbar has written extensively on the topic, highlighting a range of corroborating evidence. For example, among monkeys and apes, the size of the neocortex—the part of the brain involved in higher thinking and advanced cognitive functions—correlates with group size. Brain size also correlates with the size of grooming networks (it’s assumed that grooming plays some role in maintaining social relationships) and the occurrence of deception. The relationship between brain size and group size has also been observed in other social mammals, such as carnivores and whales.
Does the social brain hypothesis apply to humans? Dunbar thinks so. He suggests the number of individuals a person can maintain true relationships with is limited by the programming of our brain. Using the size of the human neocortex, he has calculated that human groups should contain about 150 individuals. Among traditional hunter-gatherers, according to Dunbar, this relationship seems to hold up. Even among people living in industrial societies, the number 150 holds meaning. For example, in one study, Dunbar found that people on average send Christmas cards to a total of 150 people. The idea is that even though we may interact with hundreds, even thousands of people, we can only manage to have meaningful relationships with a limited number.
Even with all the supporting evidence, it’s hard to prove that primates, including humans, evolved large brains in response to the social challenges of group living. But that’s a problem all evolutionary explanations face—it’s nearly impossible to prove that any one factor was the reason why something evolved. The lack of definitive answers may be frustrating, but contemplating the possibilities is still fun.