One of the most dramatic differences between modern humans and our closest living relatives is our family structure. Silver-back gorilla males defend a harem of several females, with whom they have exclusive rights to mate. Male chimps vie for hierarchical dominance which leads to mating success.

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When we view modern human mating arrangements alongside those of chimps and gorillas, our capacity for strong and long-lasting pair-bonds stands out. The evolution of this proclivity to pair up was an important event in our evolution. It allowed fathers to take an interest in their own children. Pair-bonding also made it possible for extended families to live close to one another and help raising the rather helpless babies that large-brained humans tend to have.

According to Sergey Gavrilets a theoretical evolutionary biologist at the University of Tennessee in Knoxville, USA:

Pair-bonding provided a foundation for the later emergence of the institution of modern family […as well as…] wealth accumulation and inheritance.“

But how did this capacity for pair-bonding evolve?

Maybe women traded reproduction for food? Perhaps monogamy reduced the risk of infanticide posed by strange males? Or might men have chosen to guard their mates by sticking around?

Each of these ideas has some support, and it is likely that pair-bonded males benefit in each of these ways. But the transition from a competitive battle in which the strongest and most attractive males win the majority of mating opportunities to a situation where males care and provide remains difficult to understand.

The problem arises because "good” men who invest heavily in caring for their partners and families should be undermined by freeloaders who do as little as they can get away with, by belligerent men who simply vanquish the good providers, and by females who choose to mate with several different males. As a result of all of these considerations, the evolution of male care doesn’t progress very far.

In a paper in yesterday’s Proceedings of the US National Academy of Sciences, Gavrilets models several different scenarios for the evolution of pairing, including mate guarding, a transactional approach in which males trade food for mating, and the capacity for communal care of offspring. He finds that each of these scenarios is undermined by the fact that females should sneak off and mate with the best males, and that males should do best by simply competing for mating opportunities rather than investing in providing and caring.

At least until he built variation among males in quality into his models. He found that otherwise unattractive and non-dominant men could raise their mating success by investing in provisioning of females who mate faithfully with them. Male provisioning, female preference for provisioning males and female faithfulness co-evolved until all but the top-ranked males in a society were good providers.

What excites me about this model is the importance of low-ranked males - the kind who never make it to the top in chimp or gorilla societies. As Gavrilets puts it:

The model shows that such a sexual revolution could have been initiated by low-ranked males who started provisioning females to get matings; after the process got underway, it would lead to a kind of self-domestication, and the end result is a group living species comprised of provisioning males and largely faithful females.

This paper only shows that the evolutionary scenario described is plausible - not that evolution necessarily played out in this way. But like all good models it makes new predictions that other scientists (in this case anthropologists) can test.

Monogamy, polygyny and promiscuity

The study of human mating systems has been shaken from a bit of a rut by recent publications. Last year in Sex at Dawn Christopher Ryan and Cacilda Jetha argued that for most of our history, people mated in largely promiscuous, though not indiscriminate fashion. Polygyny and monogamy, they argued, were mostly a consequence of the capacity to accumulate wealth and property.

This paper suggests that pair-bonds (which are important in monogamy and some forms of polygyny) may have it easier for our ancestors to own property and transfer wealth to descendants. I expect Gavriltes’ models to stimulate a lot of creative work disentangling cause from effect.

Gavrilets’ models also show that individuals tend to do better, on average, in societies in which pair-bonds become common than in societies characterised by dog-eat-dog competition among men. This prediction reinforces the emerging view [see also here and here] that more equal distribution of mates among men lead to more peaceful societies with lower levels of violence and stronger economic performance.