Family life may not have evolved without women sidling up to sugar daddies early in human history, a study suggests.
Faithfulness to men who were seen as good providers sparked a sexual revolution that replaced promiscuity with pair bonding, it is claimed.
The change, many thousands of years ago, laid the foundations of the modern family, according to a US evolutionary biologist.
Servey Gavrilets, a professor at the University of Tennessee-Knoxville, demonstrated the effect of less attractive males "buying" their way into mating opportunities.
His mathematical model showed that over time this led to a process of "self-domestication". Women had an incentive to seek out the best male providers, while it made sense for men to bond with faithful females.
The result was a transition away from a "free for all" in which rival men fought over promiscuous females.
Stability provided by long-term pair bonding between provisioning males and faithful females eventually gave rise to family living.
Explaining his theory in the journal Proceedings of the National Academy of Sciences, Gavrilets writes: "After females start developing preferences for being provisioned, the low-ranked males’ investment start to pay off.
"In the model presented here, male provisioning and female faithfulness co-evolve in a self-reinforcing manner. At the end, except for a small proportion of the top-ranked individuals, males invest exclusively in provisioning females who have evolved very high fidelity to their mates.
"Overall, females are not predicted to become completely faithful, but rather, the level of their faithfulness is expected to be controlled by a balance between selection for better genes (potentially supplied by top-ranked males) and better access for food and care (provided largely by low-ranked males).
"The establishment of pair bonding shifted competition between males for mates, which was potentially destructive for the group, to a new dimension which is beneficial for the group: competition to be a better provider to get better mates.
"Pair bonding provided a foundation for the later emergence of the institution of modern family."