In early human evolution, when faithful females began to choose good providers as mates, pair-bonding replaced promiscuity, laying the foundation for the emergence of the institution of the modern family, a new study finds.

The study helps answer long-standing questions in evolutionary biology about how the modern family, characterized by intense, social attachments with exclusive mates, emerged following earlier times of promiscuity.

In addition to the establishment of stable, long-lasting relationships, the transition to pair-bonding was also characterized by a reduction in male-to-male competition in favor of providing for females and providing close parental involvement.

The study demonstrates mathematically that the most commonly proposed theories for the transition to human pair-bonding are not biologically feasible. However, the study advances a new model showing that the transition to pair-bonding can occur when female choice and faithfulness, among other factors, are included. The result is an increased emphasis on provisioning females over male competition for mating.

The effect is most pronounced in low-ranked males who have a low chance of winning a mate in competition with a high-ranked male. Thus, the low-ranked male attempts to buy mating by providing for the female, which in turn is then reinforced by females who show preference for the low-ranked, "provisioning" male, according to author Sergey Gavrilets, associate director for scientific activities at the National Institute for Mathematical and Biological Synthesis and a professor at the University of Tennessee-Knoxville.

"Once females begin to show preference for being provisioned, the low-ranked males' investment in female provisioning over male-to-male competition pays-off," Gavrilets explained.

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 said.

The study reveals that female choice played a crucial role in human evolution and that future studies should include between-individual variation to help explain social dilemmas and behaviors, according to Gavrilets.

Gavrilets S. 2012. Human origins and the transition from promiscuity to pair-bonding. Proceedings of the National Academy of Sciences.