Sex: Monogamy may have its roots in human evolution

Posted by Katherine Marrone on Tuesday, Mar. 12 at 8:44 pm.

It’s a controversy that has instigated much discussion between scientists, and between lovers. Are humans naturally monogamous, or not? Though it’s a heated discussion, some research may help prove the former — that monogamy is actually an evolutionary adaptation, a social shift that occurred millions of years ago.

Our understanding of sexual dimorphism, the physical differences between males and females, may help us grasp this theory. According to an article by Slate Magazine, there are links between high levels of sexual dimorphism in a species and high levels of promiscuity. Species in which males are significantly larger than females tend to be more aggressive — and more promiscuous.

Take chimpanzees, for instance. Male chimps are significantly larger than female chimps. These animals are also highly promiscuous, the males often competing with other males to spread their genes. The most dominant of the group, also referred to as the “alpha males,” win these battles the most, and are, therefore, more free to mate with as many females as they want. The lower-status males, contrarily, do not have as many sexual opportunities. In addition, these promiscuous chimps do not play any role in the rearing of their offspring — they leave it up to the females.

According to the L.A. Times, these mating behaviors differ significantly from that of humans, and humans are significantly less sexually dimorphic creatures. We form a predominantly family-oriented, pair-bonding society. Fathers, for the most part, play a large role in their children’s’ lives. However, despite today’s reality, some scientists believe the mating strategies of our ancestors could have more closely resembled chimps’ polyamorous ways, meaning there was a switch to monogamy somewhere down the line of human evolution. So when did this switch occur?

Paleontologist Owen Lovejoy may have the answer. In 2009, Lovejoy published fossil specimens of Ardipithecus ramidus, an early human-like species who lived 4.4 million years ago. In Science Magazine, Lovejoy wrote that he believes it was at that time that monogamy started to appear within our species. Sexual dimorphism decreased, as well as male-to-male competition. Bipedalism — the ability to walk on two legs — also surfaced in our species during this time, allowing males to bring the females food and better care for their children. Lovejoy believes it’s this combination of the decrease in male competitiveness and the emergence of bipedalism that promoted pair bonding within our species.

This may be due to the evolutionary advantages of social monogamy, Lovejoy thinks. Females may have preferred the lower-status males. They brought them food and helped care for the children, whereas the more aggressive, larger males were too focused on spreading their genes to aid with the children. Consequently, the children who were cared for by two parents, as opposed to one, had a better chance of survival. It is also possible that lower-ranked males might also have favored relationships with female partners that were faithful.

However, the L.A. Times wrote there are some that disagree with Lovejoy’s theory, some who argue that it oversimplifies the complication of human sexual behavior. David Buss, an evolutionary psychologist at the University of Texas, is one of those people. Though he recognizes that Lovejoy’s theory is possible, it fails to take in to account the diversity of human sexual behavior, he said, of both males and females. A wide-range of sexual human behaviors include not just long-term committed relationships, but casual sex, long-term mates with sexual partners on the side and serial monogamy.

He also stated that Lovejoy also didn’t take in to account the possibility that the ancestral transition from promiscuity to monogamy wasn’t a smooth one. There could have been another type of intermediate polygyny existent in society then, one in which males having several long-term mates was the norm.

The nature of human sexuality, though muddled and controversial, is a topic worth exploring. We are complicated species — this is for sure — but perhaps the way our ancestors behaved millions of years ago can help us better understand our ways today.