Over the past decade, primatologists' work, studying social behavior in chimps, gorillas, macaques, bonobos and baboons, has led to a unifying theory that explains not only a huge range of behavior. Also, why our brains are so big and what their most essential work is.
The theory, called the Machiavellian-intelligence or social-brain theory, holds that we rise from a lineage in which both individual and group success hinge on balancing the need to work with others with the need to hold our own amid the nested groups and subgroups we are part of.
About 15 to 20 million years ago, the theory goes, certain forest monkeys in Africa and Asia developed the ability to digest unripe fruit. This left some of their forest-dwelling cousins--the ancestors of chimps, gorillas and humans--at a sharp disadvantage. Suddenly a lot of fruit was going missing before it ripened.
To find food, some of the newly hungry primate species moved to the forest edge. Their new habitat put more food in reach, but it also placed the primates within reach of big cats, canines, and other savanna predators. This predation spurred two key evolutionary changes. The primates became bigger, giving individuals more of a fighting chance, and they started living in bigger groups, which provided more eyes to keep watch and a strength of numbers in defense.
But the bigger groups imposed a new brain load: The members had to be smart enough to balance their individual needs with those of the pack. This meant cooperating and exercising some individual restraint. It also required understanding the behavior of other group members striving not only for safety and food but also for access to mates. And it called for comprehending and managing one's place in an ever-shifting array of alliances that members formed in order not to be isolated within the bigger group.
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. 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...
Source: "The Gregarious Brain" by David Dobbs, www.NYTimes.com July 8, 2007