According to the social-brain theory, it was this need to understand social dynamics–not the need to find food or navigate terrain–that spurred and rewarded the evolution of bigger and bigger primate brains.
This isn’t idle speculation; Robin Dunbar, an evolutionary psychologist and social-brain theorist, and others have documented correlations between brain size and social-group size in many primate species. The bigger an animal’s typical group size (20 or so for macaques, for instance, 50 or so for chimps), the larger the percentage of brain devoted to neocortex, the thin but critical outer layer that accounts for most of a primate’s cognitive abilities. In most mammals the neocortex accounts for 30 percent to 40 percent of brain volume. In the highly social primates it occupies about 50 percent to 65 percent. In humans, it’s 80 percent.
According to Dunbar, no such strong correlation exists between neocortex size and tasks like hunting, navigating or creating shelter.
Understanding one another, it seems, is our greatest cognitive challenge.
And the only way humans could handle groups of more than 50, Dunbar suggests, was to learn how to talk. “The conventional view,” Dunbar notes in his book, Grooming, Gossip and the Evolution of Language, “is that language evolved to enable males to do things like coordinate hunts more effectively. . . . I am suggesting that language evolved to allow us to gossip.
–Social Brain Theory
New York Times Magazine
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