Dunbar and Aiello also suggested that our ancestors banded together to avoid predators, such as hyenas and other large carnivores, which early humans may have been particularly vulnerable to as they moved from living in trees to walking upright on the land. And to maintain all that social proximity, Dunbar and Aiello say, humans evolved language, thus fueling even more brain expansion.
Coming up with direct evidence for this hypothesis is tricky because researchers have to establish that enlarged brains and big social groups led to oversized reproductive success. Unfortunately, language and romance don’t leave a fossil record. But scientists working with baboons have demonstrated an important correlation between social relationships and reproductive fitness. A team led by anthropologist Joan Silk, now at Arizona State University, showed in 2003 that the offspring of female baboons who have stronger social bonds with their peers survive longer. Another team demonstrated a similar effect of social bonds in horses.
Recently, some researchers have gone right into the brain itself to study the relationship between brain size and sociality. They are using neuroimaging to look at the correlation between the size of key brain regions and that of social groups and networks. A study in Science found a positive correlation between the amount of gray matter (especially in parts of the brain linked to sociality) in the brains of 23 rhesus monkeys and the size of the groups they belonged to; a report this year found a link between the density of gray matter in the brains of humans and the number of Facebook friends they had.
A new neuroimaging study in humans by Dunbar and his colleagues, now being prepared for publication, found that tasks requiring the so-called Theory of Mind—the ability to understand the thinking and motivations of other people—fired up key brain regions linked to social relationships. Subjects were required to read a short story and then answer increasingly difficult true-or-false questions about the motivations of the characters. The study found that parts of the brain linked to Theory of Mind and sociality were activated at a much higher level than when subjects were asked to answer factual questions about the story. The results, Dunbar says, demonstrate that social relationships require greater processing power in the brain.
Despite this new evidence and the large amount of research the social brain hypothesis has generated over the past two decades, some researchers are not entirely convinced. They argue that the hypothesis is too narrow and simplistic and does not take enough factors into account, including the quality of social relationships rather than the sheer number of them. One alternative idea, called the “cultural intelligence hypothesis,” was recently proposed by primatologist Carel van Schaik of the University of Zurich in Switzerland and his co-workers. This hypothesis puts more emphasis on social learning, the ability to transmit information and ideas—such as about food foraging strategies and technical innovations—rather than social skills alone.
Perhaps it’s just as well that the Scarecrow never got the brain the wizard promised him. If he had, he might have spent his days—like many researchers now do—trying to figure out where it really came from.