FLICKR, _DJ_Finding food and lighting fires might explain why humans have such big brains, researchers report yesterday (May 23) in Nature.

Humans’ brains are six times as large as those of similarly sized mammals, an observation that has led scientists to ponder for decades what led to such big noodles. Studies suggest social challenges, such as cooperating to hunt, or sharing cultural knowledge spurred the expansion, but a mathematical model to explain human brain evolution finds the environment had a stronger influence.

Study coauthors Mauricio González-Forero and Andy Gardner of the University of St. Andrews developed a computer model to simulate the effects of social, environmental, and cultural challenges on brain size over time. “We were expecting social challenges to be a strong promoter of brain size,” González-Forero tells New Scientist. Surprisingly, environmental challenges won out. About 60 percent of the increase in brain size over...

If left alone to survive, humans’ brains would be even bigger, according to the model, González-Forero tells The Los Angeles Times. Increasing the cooperative challenges in the model to greater than 30 percent decreased brain size, the team found. “Cooperation decreases brain size because you can rely on the brain of other individuals and you don’t need to invest in such a large and expensive brain,” González-Forero says.

“González-Forero and Gardner are on the right track,” David Geary of the University of Missouri in Columbia tells New Scientist. But he questions whether the model accurately calculated just how challenging it is to live in groups. “Their conclusion that human brain evolution was largely driven by ecological pressures, and only minimally by social pressures, is surprising and likely premature.”

Language is another missing link in the model, Dean Falk, a brain-evolution expert at Florida State University, tells The Washington Post. González-Forero admits that the model falls short in addressing the influence cultural factors, such as language, had on expansion of brain size, but he and Gardner plan to incorporate those human traits in future work.

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