Bret Stetka in Scientific American:
Compared with other mammals, and along with those of a few other notably bright creatures—dolphins, whales and elephants among them—the brain to body-size ratios of monkeys, apes and humans are among the highest. For decades the prevailing evolutionary explanation for this was increasing social complexity. The so-called “social brain hypothesis” holds that the pressures and nuances of interacting and functioning within a group gradually boosted brain size. Yet new research suggests otherwise. A study conducted by a team of New York University anthropologists, and published Monday in Nature Ecology & Evolution, reports diet was in all likelihood much more instrumental in driving primate brain evolution. In particular, it appears that we and our primate cousins may owe our big brains to eating fruit. Much of the research exploring the social hypothesis has rendered inconsistent results. And as many in the field have noted, a number of oft-cited studies in support of the theory suffer from small sample sizes and flawed design, including out-of-date species classification. The new work is based on a primate sample more than three times larger than that used in prior studies, and one that used a more accurate evolutionary family tree.
In over 140 primate species, the study authors compared brain size with the consumption of fruit, leaves and meat. They also compared it with group size, social organization and mating systems. By looking at factors such as whether or not a particular primate group prefers solitary to pair living or whether they are monogamous, the researchers figured they should theoretically be able to determine if social factors contributed to the evolution of larger brains. And it appears they could not. Dietary preferences—especially fruit consumption—seems to have been much more influential. The researchers found that fruit-eating species, or frugivores, have significantly larger brains than both omnivores and “foliovores,” those that prefer eating leaves. “These findings call into question the current emphasis on the social brain hypothesis, which suggests larger brains are associated with increased social complexity,” explains Alex DeCasien, a doctoral candidate in anthropology and lead author of the study. “Instead, our results resurrect older ideas about the evolutionary relationship between foraging complexity and brain size.”