Monday, July 16, 2007 07/16/07-Brain Size

In today's excerpt--the evolutionary basis for increased brain size:

"To [compete for] 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 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. ...

"But as the ... groups grew, tracking and understanding all those relationships required more intelligence. 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.

"[N]o 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."

David Dobbs, "The Gregarious Brain," The New York Times Magazine, July 8, 2007, p. 46.


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