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 said.
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.
First, he stopped assuming that all males would act the same. Instead, he tested what would happen if only the low-ranking males in the group offered food to females in return for mating opportunities. These weaker males had less to lose by switching strategies because they wouldn't get very far through fighting anyway.
The other key change was realizing that these low-ranking males would select faithful females.
"When I factored those things in, then things start to happen with the formation of pair bonds," Gavrilets said. Pair-bonding ultimately swept through almost the entire group.
For all the talk of the free-love 1960s, he added, "people don't realize that the most important sexual revolution for our species happened much, much earlier — probably several million years earlier."
Owen Lovejoy, a biological anthropologist at Kent State University in Ohio, said the paper fits with his own thoughts on the evolution of monogamy.
Lovejoy, who edited Gavrilets' paper, said he had theorized for decades that monogamy could be traced to males providing food to females. In a 2009 research paper, he proposed that monogamy was already in place in a 4.4-million-year-old member of the human family, Ardipithecus ramidus, based on such features as a lack of large, slicing canine teeth that would signify a lot of male competition as well as an upright skeleton that would leave arms free to carry food.
But David Buss, an evolutionary psychologist at the University of Texas at Austin and author of "The Evolution of Desire: Strategies of Human Mating," said that although the paper offered a "plausible" explanation for what may have jump-started monogamy, it hugely simplified human sexual behavior.
Human mating behaviors, for men and women, are quite varied, he said — including not just committed, long-term pairing but a smorgasbord of other strategies such as casual sex, serial monogamy, having a long-term mate with sexual partners on the side, and combinations thereof.
The study also fails to address the possibility that males didn't move straight from promiscuity to monogamy but instead to an intermediate pattern of polygyny — guarding a number of females on a long-term basis, said primatologist Bernard Chapais of the University of Montreal.
Once polygyny was in place, it would have been much easier to move to monogamy without Gavrilets' assumptions about providing food and care, Chapais said.