Warfare may have allowed humans to evolve high intelligence and the ability to work together toward common goals, according to a new study.
The researchers also offered answers to how humans evolved strong innate preferences for cooperative behaviour, as cooperative behaviour is vulnerable to exploitation by cheaters and 'free-riders'.
A free-rider doesn't contribute or cooperate and thereby undermines the effectiveness of the group's collaborative effort, something scientists call "the collective action problem."
Thus, collaborative behaviour is expected to be rare, and indeed, in animals it is typically limited to close relatives. Humans, however, are a unique species where collaboration is widespread and not limited to relatives.
In the new study published in the Journal of Royal Society Interface, lead author Sergey Gavrilets, a professor of ecology and evolutionary biology and mathematics at the University of Tennessee, Knoxville, and NIMBioS associate director for scientific activities, developed a mathematical model that offers answers to both evolutionary puzzles.
The model showed that intelligence and cooperative behaviour can co-evolve to solve the problem of collective action in groups and to overcome the costs of having a large brain.
The research points to the types of collective actions that are most effective at hastening collaboration.
According to the model, collaborative ability evolves easiest if there is direct conflict or warfare between groups, what Gavrilets calls "us vs them" activities.
In contrast, collective activities, such as defending against predators or hunting for food, which Gavrilets calls "us vs nature" activities, are much less likely to result in a significant increase in collaborative abilities.
The study also predicts that if high collaborative ability cannot evolve, perhaps for example because the costs of having a big brain are too high, the species will harbour a small proportion of individuals with a genetic predisposition to perform individually-costly but group-beneficial acts.
"Our ability to effectively collaborate with others is largely responsible for what our species came to be," said Gavrilets.
"The big question is how this ability first evolved when there are large metabolic and physiological costs related to human brain size and when collaboration can be easily undermined by free riders.
"The model offers an answer which emphasises the role of between-group conflicts in shaping unique human features," Gavrilets said.