For the last two decades, it was believed that living in large social groups fueled the evolution of bigger brains in what anthropologists call "social brain hypothesis," gradually boosted brain our size. Yet a new research suggests otherwise. Diet seems to have been responsible for the evolution of our brains and not social group and behavior.

New York University anthropologists, published Monday in Nature Ecology & Evolution, a new study suggesting that diet played a major role in the evolution of our brains. It appears that our primate cousins and we humans may owe our brain sizes to eating fruit.

Any biologist would say that what make primates special are their big brains. Bigger brains made it possible for primates to use tools, find food and traverse complex social structures within a group. However, not everyone is convinced.

For Richard Dunbar, University of Oxford evolutionary psychologist and one of the original authors and proponent of the social brain hypothesis said diet would not be enough to instigate a selective evolutionary pressure. However, Alex DeCasien, the new study's author, collected data and social lives of more than 140 species across all four primate groups; she found that neither monogamy nor promiscuity pointed towards primate's brain size.

She also found no connection concerning social complexity or group size. The only factor that seemed to point which species have larger brains was if the primate's diet consisted primarily of leaves or fruit, Science reported. The study showed that primates who primarily ate fruits had 25-percent bigger brains than those who ate leaves.

Surprisingly, Dunbar rather agreed that eating fruit offers an animal a jolt of calories, which might account for primates' brains to benefit from the available energy. However, he asks, why was the energy directed toward developing a bigger brain, and not some other body part? According to him, that is where the primates' complex social lives come in. Better diets merely provided the fuel for evolutionary change, he said.

However, Dunbar still maintains that social group size, not diet, is the key driving force. Yet, according to Scientific American, the studies that delved on "social brain hypothesis" in the past have yielded "inconsistent results."