Monogamous, romantic love — or, more prosaically, pair-bonding — may have evolved in a sexual revolution that could have laid down the roots of the modern family, according to an intriguing new mathematical model.
Researchers have long wondered why — unlike our sexually promiscuous chimpanzee-like ancestors — humans developed strong pair bonds with individual partners. It’s thought that at one time, human ancestors did engage in chimp-like habits of sex and child-rearing, in which strong alpha males mated freely with the females of their choice, and then left the child-raising duties to them. So, the question is, How did we got from there to the modern-day monogamous, two-parent family?
“People have been discussing ways by which the transition from promiscuity to pair-bonding could have occurred and there are various different scenarios,” says study author Sergey Gavrilets, distinguished professor of ecology, evolutionary biology and mathematics at the University of Tennessee in Knoxville. “What I’ve done is shown mathematically that some of these scenarios are more likely than others.”
Gavrilets study suggests that a sexual revolution occurred, led by low-ranked males and faithful females. Low-ranked males, who had no hope of physically overcoming the dominant members of their groups, instead began providing extra food to certain females, to curry sexual favor. These females responded by remaining faithful to their breadwinning males. That change in behavior favored the reproductive success of these monogamous couples — pair-bonding offered a greater likelihood that their children, who took a lot of effort to raise, would survive — ultimately moving humanity away from a promiscuous mating system dominated by alpha and beta males.
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Gavrilets reasons that males in promiscuous hierarchical species face a dilemma because the alpha and beta males tend to get all the mating action. Lower-ranking guys have two choices: either compete their way to the top and win reproductive opportunities, or look for ways to beat the system.
“They can put effort into achieving high-dominance status. If they do, they will have more mating opportunities and more offspring,” says Gavrilets. “On the other hand, they can do something else. They can try to increase the survival of their own offspring directly and that would also increase the potential for greater numbers of surviving offspring.”
One way a male could do this is simply by guarding a particular female and making sure no other males can mate with her. Or he could exchange food for sex with a variety of females, in something like an early version of prostitution.
Alternatively, a male could provide extra food to a specific female exclusively, thereby getting more chances to mate with her and also helping her and her offspring survive with the increased nutrition. “Males get a double benefit. There’s the immediate benefit of more mating. [Having more food] also increases female fertility and decreases the interbirth interval so you can have more kids more often,” Gavrilets says.
Another male strategy could be to help out with kids directly — whether they’re his offspring or not. By helping all of the group’s mothers raise their children, the males help increase the offspring’s chances of survival, presumably including those that he fathered.
In Gavrilets mathematical models, the only scenario that appeared to push humans away from promiscuity was one in which males — low-ranking males, to be specific — provisioned one female. (Thankfully, love doesn’t seem to have its roots in the oldest profession.) “What happens is that for guys at the bottom of the hierarchy who are weak or small and who would never be able to win competitions, mate-provisioning becomes a very valuable option” says Gavrilets. “They start provisioning the females first and then females develop a preference for provisioning, and then [begins] the whole process of the co-evolution of male provisioning and female faithfulness.”
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In other words, when females come to prefer males who provide a bit more for them even if they aren’t the biggest or strongest, those males start preferring females who are faithful, and this process ultimately creates pair bonds between them.
Both male and female choice drove evolution. “Female faithfulness increases as a result of males selecting more faithful females and male provisioning grows as females select for better providers and they co-evolve in a mutually beneficial way,” Gavrilets says.
Sarah Hrdy, professor emerita of anthropology at the University of California, Davis, and a leading evolutionary theorist, commended Gavrilets’ research, though she took issue with some of the assumptions made in his mathematical model. “We don’t fully understand the evolution of human pair-bonding so I welcome this effort,” says Hrdy.
However, she disagreed with a few of its key suppositions: for one, she thinks that cooperative rearing of children by women was a critical factor in human evolution, but Gavrilets’ model relies on research that doesn’t reflect this. Gavrilets uses data that suggest that early human females left their home groups in adolescence in order to mate, and that they did not cooperate in child care with females in their new group because they were unrelated.
But Hrdy says, that “multilocal residence patterns were far more typical,” citing data that suggests that males often moved to live with a female and her family for a while. “Over the course of their lifetimes, both sexes are moving, perhaps multiple times,” she says, allowing for cooperation between females to be important.
Hrdy adds that paternal commitment in the human species is highly variable — both today an in ancient times. “Some men will do anything to remain near their children. Others, even some [who are] certain of their paternity, act like they do not even know they had children,” Hrdy says. “I don’t think human mothers in the past could count on the long-term survival and fidelity of provisioning mates any more than mothers today can. It looks to me like males are responding to a wider range of factors than can be represented in such a model.”
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Gavrilets agrees that the model is not complete and is working to include more variables. “I think what this model does is that it looks at the very first step in this very long process,” he says. It does not, however, fully explain the origins of male tendencies toward monogamy.
Whatever started it, Gavrilets notes, humans’ transition to monogamy was much more radical than the sexual revolution of the 1960s — even though it went in the opposite direction. “Not many people realize that the most important sexual revolution for our species probably happened several million years ago,” Gavrilets says. “This revolution was accomplished by the masses of lower-ranked males, along with females, directly against the elite of alphas, so the term revolution is even more appropriate.”
The study was published in the Proceedings of the National Academy of Sciences.
Maia Szalavitz is a health writer at TIME.com. Find her on Twitter at @maiasz. You can also continue the discussion on TIME Healthland’s Facebook page and on Twitter at @TIMEHealthland.