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a number of oft-cited studies in support of the theory suffer from small sample sizes and flawed design, including out-of-date species classification.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.”

DeCasien is referring to research begun in the 1970s by University of California, Berkeley, anthropologist Katharine Milton who studies the dietary ecology of primates. Her work comparing spider and howler monkeys suggests foraging for fruit is associated with larger simian brains, more than eating leaves. It is possible the nutrients in fruit may have helped increase primate brain size, in which case DeCasien envisions an evolutionary feedback loop. “Fruits are a higher-quality type of food relative to leaves. Since the brain is so energetically expensive, higher-quality food is a necessary part of covering the costs of evolving a larger brain,” she says. That larger brain is in turn more capable of scouring, say, the canopy of a tropical forest for even more fruit.

DeCasien endorses the validity of Milton’s suggestion that the complexities of fruit foraging may have also driven primate brain evolution. Being able to find, pick and peel a dangling passion fruit takes a lot more brainpower than simply tearing off leaves. In this case, a bigger brain would come in handy—in fact, it would be selected for.

Anthropologist Karin Isler of the University of Zurich, who studies brain evolution in animals and was not involved in DeCasien’s latest study, is impressed by the new findings. Yet she believes more research is necessary to parse all the factors that could be at play. “Diet composition is only one factor which correlates with relative brain size,” she explains. “There is also seasonality, manipulative skills, extractive foraging, age at first reproduction, reproductive rate and longevity, to name a few. And it would be interesting to study how all of these factors [might work together.]”

Evolutionary biologist Robert Barton of Durham University in England, who has a study similar to DeCasien’s under review but was not involved in her work, also believes other factors may be involved. “Where we agree is that there is little or no evidence of a “social brain” effect in these large data sets, and that there are more robust correlations with ecological variables,” he says. “However, our analyses do not identify [fruit eating] as a robust correlate of brain size when we consider a wider range of variables. I would not be tempted into speculations about a role for specific nutrients in brain evolution.”

What is clear: the pendulum of brain evolution theory seems to be swinging back toward ecological influences like diet and away from social explanations. “I have been very skeptical about the social brain hypothesis from the outset,” says Robert Martin, a biological anthropologist and emeritus curator of biological anthropology at The Field Museum in Chicago. “As far as I am concerned, the new paper by DeCasien and colleagues effectively eliminates that theory. It convincingly shows that there is no meaningful association between brain size and social organization whereas there is good evidence for a relationship between brain size and diet.”

As DeCasien acknowledges, her new findings echo another longstanding anthropological debate that mulls how the awesomely complex human brain evolved from its primate predecessor. Many experts cite seafood. As we began eating more fish and combing the shores for calorically rich shellfish beds, our brains were suddenly bathed in neurologic nutrients like omega fatty acids, which allowed for rapid growth and increased complexity. To DeCasien this suggested another Darwinian feedback loop. Yet again the question arises: To what degree was it eating seafood that helped evolve our hefty brains, set against the activities it took to do so—the fishing, the tracking of tides and the complexities of prehistoric oyster shucking?

Future research should further trace the primate brain’s 60-million-year natural history, and hopefully determine which factors presented the opportunity for a bigger brain to arise—and which were the result of having one.