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WHY did our primate ancestors get so smart so fast? The most popular idea is that social living was the key to shaping our larger brains. Now this is being challenged by a study which suggests that solving the day-to-day problems of finding food had just as big an effect.

鈥淲e should perhaps reconsider the view that a single selection pressure favoured enhanced brain size,鈥� says evolutionary biologist Simon Reader at Cambridge University. His study, conducted with Kevin Laland, suggests that a range of factors, not just the demands of group living, favoured bigger brains.

There are two main theories of what propelled the massive expansion of our ancestors鈥� brains. One blames it all on society. Living in extended family groups makes it easier to defend feeding sites and spot predators. But predicting how others will react to different situations and keeping track of alliances and rivalries requires extra brainpower. The other theory says ecological challenges were more important: using a rock as a tool to crack a nut or remembering which trees are in fruit at a particular time of year is easier if you are smarter, regardless of whether you live in a group.

Reader and Laland pooled data from around 1000 published studies on 116 primate species. By looking at different species鈥� tool use, their innovative behaviours and how individuals learned from others, they set out to discover if any characteristics of social life or the environment were particularly associated with cleverness.

Their analysis confirms that having a bigger brain makes you generally smarter. But no single factor stood out as being more important than the rest. Crucially, primates living in larger groups were no better than others at learning from their peers.

This suggests that group living may not be so special after all. 鈥淚t is likely that multiple sources of selection favoured the evolution of large brains,鈥� says Reader.

Not everyone agrees. 鈥淥nce you鈥檝e got the machinery in place, it鈥檚 really a matter of software design to apply it to other things,鈥� says evolutionary psychologist Robin Dunbar of Liverpool University. He thinks the results are still consistent with the idea that social intelligence came first, with other skills and abilities following later.

Reader admits that he can鈥檛 say for certain whether ecological intelligence or social intelligence came first. But he points out that most of the innovative behaviours seen by previous researchers concerned finding food, not dealing with others. 鈥淚 do not think we can neglect ecological demands.鈥�

But Robert Seyfarth of the University of Pennsylvania argues that, unlike other animals, primates鈥� most impressive intellectual talent is their ability to form complex social relationships. He suggests that other abilities considered by Reader and Laland could have links to social skills that aren鈥檛 obvious. 鈥淭his study will make us think about what exactly we mean by 鈥榮ocial intelligence鈥�,鈥� he says.