"Those who have most closely studied the subject believe that communal marriage was the original and universal form throughout the world." - Charles Darwin
Evolutionists know well that the keystones of survival (food, sleep, cognition) develop great complexity over evolutionary time. But as the all-important mechanism of genetic replication, sex is especially complicated.
In the world of sex, we find tremendous variation in behaviors and social formations. This is true among humans as well as nearly every other animal species on earth. Dogs hump fire hydrants and other species. Giraffes and bonobos exhibit a dearth of homosocial and homosexual behavior. Oysters are biologically transgender. Humans are not a far cry.
To classify normal or standard human sexual behavior poses a challenge, because we are pitted somewhere in between two opposing forces: monogamy on the one hand and polyamory on the other. That is, we yearn for a deep primary pair-bond yet at the same time lust for others outside those bonds and relish in the novelty of new sexual partners. (It should go without saying that this varies in degree between genders and between persons, but is generally true of most people.)
Though anthropologists and evolutionary psychologists have already seemed to nail down the science of human sexual behavior, some call it into question, perhaps most popularly by Christopher Ryan and Cacilda Jetha in their controversial science book, Sex at Dawn, which rejects the "standard narrative" of sexual behavior and posits a fluid, promiscuous one where "most mature individuals would have had several ongoing sexual relationships at any given time."
Dawn paints a picture of humans in the ancestral environment that have sex for multiple coinciding reasons, all of them secondary to the central reason: to bond. That sex is a social bonding mechanism is something that is at first obvious, yet its implications are profound. Think of the human animal living in a small group struggling to survive in the wild. This is arguably the most social species the Earth has ever seen, more cohesive, emotional, and structured than even the colonial insects. In this light, bonding has renewed importance, because there are only about a dozen ways for humans to bond with one another, and of those, sex is by far the most potent. On this, few would disagree.
In fact, it is this bond that gauges the validity of the sex act in almost all cultures. If a woman has sex for money, she's a whore. If she has sex for pleasure, she's a slut. It is only when a woman has sex for love that the sex act is validated by cultural conventions. From a scientific perspective, love can only be understood as the bond between clansmen. Thus, it is the bond of love that drives natural human sexual behavior.
To expand upon this powerful idea once more, consider the commonly accepted idea in the biological sciences that women exchange sex for male resources in the ancestral environment, as if relationships in the tribe were like that of a modern marketplace where rational players exchange goods and services numbly. But the tribe is not a market full of rational strangers. It is a cohesive group of emotional allies. Thus, it makes more sense that men gave women resources out of love, a respect for the bond. Furthermore, women would give resources to men based on this bond and - gasp! - could it be so radical a notion that men and women had sex based on the bond of love?
This topic, bonding, takes place after or aside from mate selection, which is complicated by things like genetic value, social status, and fertility. But can it be doubted that once bonds are formed they play a central role in the "exchange"? This is one more way that acknowledging the social nature of our species helps to understand human behavior.
The controversial idea of polyamory is based on the uncontroversial idea that cohesive groups fare better in the wild. Why did many of the other human species that co-existed with Homo sapiens in the Pleistocene die out? We don't know, but a good guess is that they lacked the unity of Homo sapiens groups. Clearly, the more bonds in a group and the deeper those bonds, the more cohesive the group.
The possibility of polyamory in the ancestral environment is diminished by the futility of polyamory in the societal one. Today, we live in a world of strangers, taught selfishness and rugged individualism and ownership of everything, even women and men, and let's not forget about the blight of religion that underscores most cultural tradition and sexual repression in all of its pathological forms. Compare this to the wild tribe where sharing is religion and ownership is communal, where the group means everything, and strangers are dangerous.
When matched to the goals of happiness and/or building deep, lasting personal relationships, effective polyamory is simply much harder to achieve than the standard practice of monogamy. These relationships are perhaps naturally unstable, as the wild intended, and unreliable in the long term, even more so than the quandary that is standard monogamy.
In fact, polyamory explains exactly why monogamy works in society. Polyamory predicts a strong primary pair-bond, probably a secondary pair-bond, and maybe tertiary or quarternary pair-bonds. If you cut it off after the primary, then you've got yourself monogamy. The cost of losing those other relationships is the reward of stabilizing the primary one, a dire need in society.
In society, we need more security than humans in the wild. That rugged individualism leaves us feeling alone. That selfish ownership-based social economy makes us greedy. That world of strangers makes us insecure and jealous. Without marriage, what would our primary pair-bonds be? How would we escape the classic human fear of abandonment when we are trapped in a tribe of two?
The validity of polyamory is not lost on me. There are people who may thrive on its merits, who have for whatever reason escaped the "turgid slick of conformity," as Russell Brand calls it. These may be modern sluts and whores, but also serial daters, Don Juans, cat ladies, desperate husbands, repressed wives, swingers, and polyamorists. Evolution predicts diversity here, and isn't that the whole point?
