Women's preference for providers rather than fighters led to the emergence of the modern family, say researchers.
Early in human evolution, promiscuity was replaced by pair-bonding, and there was a move away from male-to-male competition in favor of providing for females and providing close parental involvement.
But, says the team, they've demonstrated mathematically that the most commonly proposed theories for the transition to human pair-bonding aren't biologically feasible.
Instead, they say, the transition to pair-bonding was based on female choice and faithfulness, so that providing for females became a better way of getting sex than competing with other males.
The effect is most pronounced in low-ranked males who would have a poor chance of winning a mate in a fight.
"Once females begin to show preference for being provisioned, the low-ranked males' investment in female provisioning over male-to-male competition pays-off," says Sergey Gavrilets of the National Institute for Mathematical and Biological Synthesis and a professor at the University of Tennessee-Knoxville.
Gavrilets says that the study's results describe a 'sexual revolution' initiated by low-ranking males who began providing in order to get matings.
"Once the process was underway, it led to a kind of self-domestication, resulting in a group-living species of provisioning males and faithful females," he says.