Ask any biologist what makes primates special, and they’ll tell you the same thing: big brains. Those impressive noggins make it possible for primates from spider monkeys to humans to use tools, find food, and navigate the complex relationships of group living. But scientists disagree on what drove primates to evolve big brains in the first place. Now, a new study comes to an unexpected conclusion: fruit.
“The paper is enormously valuable,” says Richard Wrangham, a biological anthropologist at Harvard University who was not involved in the work. For the last 20 years, many scientists have argued that primates evolved bigger brains to live in bigger groups, an idea known as the “social brain hypothesis.” The new study’s large sample size and robust statistical methods suggest diet and ecology deserve more attention, Wrangham says. But not everyone is convinced. Others say that although a nutrient-rich diet allows for bigger brains, it wouldn’t be enough by itself to serve as a selective evolutionary pressure. When the authors compare diet and social life, “they’re comparing apples and oranges,” says Robin Dunbar, an evolutionary psychologist at the University of Oxford in the United Kingdom and one of the original authors of the social brain hypothesis.
Alex DeCasien, the new study’s author, didn’t set out to shake up this decades-long debate. The doctoral student in biological anthropology at New York University in New York City wanted to tease out whether monogamous primates had bigger or smaller brains than more promiscuous species. She collected data about the diets and social lives of more than 140 species across all four primate groups—monkeys, apes, lorises, and lemurs—and calculated which features were more likely to be associated with bigger brains. To her surprise, neither monogamy nor promiscuity predicted anything about a primate’s brain size. Neither did any other measure of social complexity, such as group size. The only factor that seemed to predict which species had larger brains was whether their diets were primarily leaves or fruit, DeCasien and her colleagues report today in Nature Ecology & Evolution.
That doesn’t actually surprise Dunbar. “In order to have a bigger brain, you have to have a change in diet,” he says. Leaves’ nutrients are locked up behind thick cell walls, and breaking down those barriers takes a lot of time and energy. Primates that eat leaves have to lie around for hours, with all their energy going toward digestion. Eating fruit, on the other hand, offers an animal a jolt of calories in an easy-to-digest package. In primates, the main beneficiary of all that newly available energy is the brain.
So far so good. But for Dunbar, a key question remains: Why was that energy directed toward developing a bigger brain, and not some other body part? That’s where primates’ complex social lives come in, he says. Living in big groups makes it easier for primates to fend off predators, but it also means managing increasingly complex social relationships. The cognitive demands of those relationships made bigger brains the best use of the extra fruit-derived energy. Better diets merely provided the fuel for that evolutionary change. “[Diet and sociality] are not alternative explanations” for larger brains, Dunbar says. “They are complementary explanations.”
DeCasien sees another possibility, one that keeps fruit-eating in the driver’s seat. Eating fruit is more cognitively challenging than eating leaves, she says. A primate can find leaves basically anywhere, but it must remember where and when the best fruit is likely to grow. Fruit eaters also range over larger areas than leaf eaters, so they need top-notch navigation skills. And because some fruits may be hard to reach or protected by defenses like spines, primates also need problem solving skills or even tools. Evolution could have pushed fruit-eating primates to develop bigger brains to deal with these complex foraging conditions, DeCasien says. In that case, social life might be largely irrelevant.
DeCasien admits the answer might not be black and white. Diets could have jump-started brain growth, enabling the first inklings of more complex social lives. The cognitive demands of those social lives, in turn, could have further driven evolution. “It’s definitely impossible to tease apart at some point,” she says.
Any of these stories are plausible, Wrangham says, but it’s notoriously hard to distinguish selective pressures from beneficial physiological changes in correlation studies like this one. He suspects that diet allowed, rather than drove, the evolution of big brains. But he’s convinced that diet is intimately tied to evolution, especially in a particular species of primate: humans. “Cooking is what has taken the human lineage into a totally new realm,” he says, especially after we learned to cook meat. The new study supports this history of diet-linked cognitive leaps, he says, and he hopes it will bring renewed attention to diet’s role in evolution.