Asked by: Lizzie Alton, Glasgow
One possibility is that large brains are sexier. The person that can make music and art, or tell stories, may be more attractive to potential mates.
But in the 1990s, anthropologist Robin Dunbar suggested that humans might also need large brains to keep track of their complicated social lives. Human social circles normally comprise around 150 people, compared with 50 for chimpanzees. Larger social groups have exponentially more interrelationships and our survival and success depends on being able to react to and predict the behaviour of our peers. Related to this is the idea of social dominance. Once our ancestors had begun to master their environment, their biggest threats were other humans. Leadership tussles within and between tribes favoured smarter humans much more than those that were just stronger.