LOS ANGELES -- The roots of the modern family -- monogamous coupling -- lie somewhere in our distant evolutionary past, but scientists disagree on how it first evolved.
A new study says we should thank two key players: weak males with inferior fighting chops and the females who opted to be faithful to them.
These mating strategies may "have triggered a key step in the very long process of the evolution of the family," said study author Sergey Gavrilets, a biomathematician at the University of Tennessee in Knoxville. "Without it, we wouldn't have the modern family."
The mating structure of humans is strikingly different than that of sexually promiscuous chimps, in which a few alpha males dominate other males in the group and, by dint of their superior fighting prowess, freely mate with the females. Lower-status males are largely shut out from mating opportunities.
In addition, male chimps don't contribute to rearing their young -- that is left to the female.
Some scientists believe that ancestors of humans had chimp-like patterns of mating and child-rearing. The transition to pair-bonding was a key step for our big-brained species, because our children take years and much energy to raise to independence. It's hard for a mother to go it alone.
How did the transition take place? It's not a simple question, Gavrilets said.
Dominant, promiscuous males have it good -- they don't have to invest in their young because they'll have plenty of offspring regardless, Gavrilets explained.
Males that help feed and protect a smaller number of offspring can also be very successful, reproductively speaking -- but only if they can be sure who their children really are or if they provide for all the young in a group. Otherwise, the "providers" will be wasting their resources on offspring that are not their own, and there is ample opportunity for some males to cheat and not do their part.
Gavrilets wanted to see how we might have gotten from A to B. In his work published Monday in the Proceedings of the National Academy of Sciences, he used mathematical models to test factors that scientists believe may have driven the transition to pair-bonding. These include mate-guarding (males hang around the females they've mated with so others cannot mate with them too) and provisioning (males offer food or other resources to a female in return for sexual favors).
His number-crunching found that these factors alone were not enough to move a species away from promiscuity. The models did work, though, with a few adjustments.