Tough conditions are responsible for making the human brain unusually large, says a study which contradicts the notion that we evolved bigger brains to cope with complex social relationships.
The study, published in the journal Nature, sheds light on a mystery long cast over the story of human evolution.
Researchers at the University of St Andrews in the UK found that when human ancestors cooperated to solve problems, this made brains smaller as working together allowed human beings to save investing resources in the brain.
"The findings are intriguing because they suggest that some aspects of social complexity are more likely to be consequences rather than causes of our large brain size," said Mauricio Gonzalez-Forero from University of St Andrews.
"The large human brain is more likely to stem from ecological problem-solving and cumulative culture than it is from social manoeuvring," said Gonzalez-Forero.
For decades debate has raged over why the human brain has evolved to be so unusually large.
A number of theories exist including the 'social brain hypothesis', which suggests that bigger brains evolved to help manage our increasingly complex social lives.
The researchers created a new mathematical model to ascertain which situations are actual causes, rather than just side effects, of large brains.
It mechanistically models the energy costs of brain growth and maintenance and the brain's ability to enable its bearer to solve environmental and social problems.
The study found that human-sized brains and bodies can evolve when individuals live in tough environments, engage in lots of cooperation, and undergo a reasonable amount of between-group conflict.
However, in contrast to current understanding, the study found that it is tough environments in particular that expand brain size, provided that individuals can keep improving their skills through their youth.
Such sustained improvement of the individuals' skillsets as they age may be facilitated by cultural processes, ie, learning things that previous generations have figured out rather than figuring them out for themselves.
The study concluded that a combination of difficult environments and cultural processes likely caused human brain expansion.
The effect of cooperation and between-group competition is not to increase brain size, but actually to decrease it, researchers said.
The reason is that, by cooperating, individuals can rely on each other's brains and so can save resources by growing smaller brains themselves, they said.