9 August 2004

Big Brain Evolved Through Social Problem Solving

by Kate Melville

Why did humans evolve such large brains? While humans have an unusual array of characteristics that distinguish us from other species, it is our cognitive abilities and open-ended thinking that are most remarkable. University of Missouri-Columbia researchers found that the ability to excel at social problem solving led human brains to surpass other species in size, developing ecological dominance 1.5 to 2 million years ago.

Researchers Mark Flinn, Carol Ward and David Geary have integrated this new theory of human intelligence evolution with recent developments in the fields of paleoanthropology, cognitive psychology and neurobiology. Their study will appear in an upcoming issue of Evolution and Human Behavior. Through testing fossils for brain size, body size and archaeological evidence of behavior, along with recent comparisons of our mental abilities to those of apes, the researchers have found support for a theory proposed by zoologist Richard Alexander that humans evolved large brains to negotiate and manipulate complex social relationships.

"Most traditional theories, including that of Charles Darwin, suggested some combination of tool use and hunting were the key selective pressures favoring big brains, but increasing evidence of hunting and tool use in other species such as chimpanzees indicates our ancestors were not unique in that regard," Flinn said. "The most exceptional of our mental gifts involves understanding what is going on in other people's minds by using skills such as empathy and self-awareness."

Ward says the hominid brain increased 250 percent in less than 3 million years, particularly in the neocortex area that controls cognitive development. She said scientific evidence says absolute, not relative, brain size is more closely tied to intelligence. She credits the increasing importance of complex social coalitions with the human brain's evolution. "Great apes did have a lot of interaction with non-kin, seeking subordinate leverage and depending on the good graces of others, but hominids not only competed against other hominids, they competed against other groups much like sports teams do today," Ward said. "Eventually, competing against other hominids became the driving force and, at some point, became a factor in ecological dominance."

Geary said the ability to think ahead and mentally simulate what others might do is another reason humans achieved dominance.

"As our ancestors gained control of other species, through hunting and other means, the most important impediment to their ability to survive, reproduce and thrive was the competing interests of other people," Geary said. "And so it remains."

"We think this model explains the data better than any other model. The tests available, although not comprehensive, certainly support it and provide a better explanation than the other ideas out there," said Ward.