Working together to achieve a common goal may have helped us develop bigger brains compared to our loner ancestors, a new study reveals.
Lead author Luke McNally of Dublin's Trinity College used computer simulations to show that neuron networks actually grow, over many generations, as a result of teamwork.
This is because co-operation works the brain as it keeps track of ever more complicated social interactions.
Meanwhile, evolution favours more intelligent individuals as they are likely to attract more mates.
"We co-operate in large groups of unrelated individuals quite frequently, and that requires cognitive abilities to keep track of who is doing what to you and change your behaviour accordingly," the Daily Mail quoted McNally as telling a foreign new agency.
And we do this because co-operation benefits us in the long run.
"If you co-operate and I cheat, then next time we interact you could decide: 'Oh well, he cheated last time, so I won't co-operate with him.' So basically you have to co-operate in order to receive co-operation in the future," he added.
McNally's computer simulation involved two scenarios, one where two detained criminals had to decide whether to pin the blame on the other, and one where two people trapped in a car in a snowdrift worked out if they should both dig their way out, or let the other person put in the spade work.
In each case, team work had the least benefit for the individual, but the researchers found that the bigger the brain got, the more likely it was that co-operation was seen as the way forward.
Robin Dunbar, an evolutionary anthropologist at Oxford University, told the news agency that our brain size limits the number of people in our personal social network that we can deal with.
At the moment, that number is about 150. To cope with, say, 500, our brain would need to double in size.
If we wanted to cope with the whole planet, we'd need house-sized grey matter.
The study appeared in Proceedings of the Royal Society B.