There are bones hidden away in almost every cupboard in many of the rooms of New York University's primatology department, and James Higham is keen to explain to me what they can tell us about an important part of our evolution: why we have such big, heavy brains.
He shows me hordes of lemur skulls, as well as casts of our extinct relatives.
Of particular interest to him are the sizes of their braincases. After studying this feature in primates including monkeys, lemurs and humans, he and his colleagues have presented an intriguing new idea as to why our brains are so large.
Orangutans live in very small groups (Credit: Mervyn Rees/Alamy)
The reason why some primates have bigger brains than others is often said to be their social behaviour. That is, primates that move around in bigger and more complex social groups require bigger brains in order to efficiently manage all of those social relations.
The new analysis found that diet – not social group size – was the key factor linked to brain size
This theory has been around for over two decades, and is called "the social brain hypothesis".
Following a large-scale analysis of primates, Higham and his colleague Alex DeCasien are confident that the social brain theory does not tell the whole story.
Rather, brain size is more accurately predicted by primates' diet, according to their new study published in the journal Nature Ecology and Evolution.
To come to this conclusion, the team, led by DeCasien, put together a dataset of 140 primate species, including animals like the aye-aye and several species of gibbon. This allowed them to study the relationship between the size of primate brains and several social factors, such as group size and social structure.
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