Monogamy - Humans - Evolution of Monogamy in Humans - Evolutionary History of Human Monogamy

Evolutionary History of Human Monogamy

The date when monogamy evolved in the human lineage is heatedly debated with differing views from within the field of paleoanthropology and from genetic studies. Ultimately, there are two prevailing views on the evolutionary history of monogamy in humans: monogamy evolved very early in human lineage and monogamy evolved much more recently, less than 20,000 years ago.

Paleoanthropological estimates of the time frame for the evolution of monogamy are primarily based on the level of sexual dimorphism seen in the fossil record because, in general, the reduced male-male competition seen in monogamous mating results in reduced sexual dimorphism. According to Reno et al., the sexual dimorphism of Australopithecus afarensis, a human ancestor from approximately 3.9–3.0 million years ago, was within the modern human range, based on dental and postcranial morphology. Although careful not to say that this indicates monogamous mating in early hominids, the authors do say that reduced levels of sexual dimorphism in A. afarensis “do not imply that monogamy is any less probable than polygyny”. However, Gordon, Green and Richmond claim that in examining postcranial remains, A. afarensis is more sexually dimorphic than modern humans and chimps with levels closer to those of orangutans and gorillas. Furthermore, Homo habilis, living approximately 2.3 mya, is the most sexually dimorphic early hominid. Plavcan and van Schaik conclude their examination of this controversy by stating that, overall, sexual dimorphism in australopithecines is not indicative of any behavioral implications or mating systems.

The genetic evidence for the evolution of monogamy in humans is more complex but much more straightforward. While female effective population size (the number of individuals successfully producing offspring thus contributing to the gene pool), as indicated by mitochondrial-DNA evidence, increased around the time of human (not hominid) expansion out of Africa about 80,000–100,000 years ago, male effective population size, as indicated by Y-chromosome evidence, did not increase until the advent of agriculture 18,000 years ago. This means that before 18 000 years ago, many females would be reproducing with the same few males.

Scientists discuss the evolution of monogamy in humans as if it is the prevailing mating strategy among Homo sapiens, although only approximately 17.8% (100) of 563 societies sampled in Murdock’s Atlas of World Cultures has any form of monogamy. These societies with monogamy account for much larger than 17.8% of the World population. Therefore, “genetic monogamy appears to be extremely rare in humans,” and “social monogamy is not common, ... often reduc to serial polygyny in a biological sense”. This means that monogamy is not the predominant mating system among the hominid lineage and probably never was.

In Herodotus's Histories, which contains some of the earliest anthropological writings, Herodotus writes about a few societies and tribes that did not opt for social monogamy at the time (circa 500 BC). One tribe he mentions had open relationships in the villages and after puberty the boys were assigned their 'fathers' according to whom they most resembled. He mentions other socially open tribes where mating openly outside in the daylight was observed. It is postulated that the reason he notes these tribes is because they were not the norm in Ancient Greece, where monogamy prevailed at that time.