A high-fruit diet is known to keep people healthy, but it may also have helped us develop into humans, a new study suggests.
Scientists have discovered a link between the amount of fruit eaten by primates and the size of their brains.
Previously it was thought that the larger brains of monkeys, apes and humans developed to cope with the complex social maneuverings required to successfully live in a group, a theory known as social brain hypothesis.
For example primates need substantial brainpower to understand who their friends and enemies are, and keep track of ever-changing hierarchies and power struggles.
But researchers at New York University believe primates and humans actually ate their way to a bigger, more complex brain.
“Are humans and other primates big-brained because of social pressures and the need to think about and track our social relationships, as some have argued?" said Dr James Higham, an assistant professor in NYU’s Department of Anthropology.
"This has come to be the prevailing view, but our findings do not support it. In fact, our research points to other factors, namely diet."
The team compiled the biggest ever database of more than 140 different species to explore the relationship between brain size, different kinds of social behaviour and feeding habits.
Previous studies investigating brain size evolution in primates found a correlation between the average number of group members and the size of their brain relative to body size. But the researchers found that the relationship vanished when more complex social behaviour – such as monogamy – was added into the mix.
They found no link between brain size and any measure of sociality, but they did find there was a strong link to diet. Fruit-eating primates have around 25 per cent more brain tissue than plant-eating species.
The researchers suggest that the bigger brains probably evolved to recall fruit locations, and work out new ways to extract flesh from tough skins. Fruits also contain for more energy than plants, giving brains a boost.
"Fruit is patchier in space and time in the environment, and the consumption of it often involves extraction from difficult-to-reach-places or protective skins," said doctoral student Alex DeCasien, the lead author.
"Together, these factors may lead to the need for relatively greater cognitive complexity and flexibility in fruit eating species.
"Complex foraging strategies, social structures, and cognitive abilities, are likely to have co-evolved throughout primate evolution.”
"However, if the question is: ‘Which factor, diet or sociality, is more important when it comes to determining the brain size of primate species?’ then our new examination suggests that factor is diet."
British experts said the new study had turned evolutionary biology on its head.
Dr Chris Venditti, of the School of Biological Sciences at the University of Reading said: “There is a long-standing notion that social complexity is linked to cognitive complexity.
“DeCasien et al may have delivered a blow to social brain hypothesis that has it reeling, and if future work irons out some of the remaining methodological creases, it may be down and out. Then we will be left with the extraordinary position of trying to explain primate cognition without sociality.”
The research was published in the journal Nature Ecology and Evolution.