Want to justify the amount of time you spend on your online dating profile? It turns out that monogamy (along with language, booze, cooking, and bipedalism) may be one of those unique traits that "made us human". While primates as a whole are an unusually monogamous for mammals, our closest relatives, the great apes, are all into promiscuous free-love. Though the benefits of the human pair bond are obvious now – it's helpful for rearing big-brained, energy-intensive offspring – scientists are still split on why human monogamy evolved in the first place.
The chimp, bonobo and human lineages split about 7 million years ago; indirect evidence of monogamy in our very early ancestors emerges at around 3.5 million years ago. One theory holds that primate monogamy emerges when females are too spread out for a male to successfully mate with more than one, which seems to go against the fact that humans and apes always live in groups. The other potential driver of monogamy – the threat of infanticide – suggests that our ancestors spent a lot of time killing off each other's babies. A new computer simulation has supposedly conclusively demonstrated that it was indeed infanticide that drove early humans into each other's arms. In this simulation, paternal care is the result of monogamy, not the driver of it.
What the study doesn't seem to explain, however, is why sexual pair-bonds (sometimes known as "love") exist in humans but not in gorillas, chimps or bonobos. While extremely rare in matriarchal bonobo society, infanticide is a real and common risk for chimp and gorilla mothers. Why do these species have a harem model, where one or two dominant males mate with a group of females, while we have a couple-based model? Perhaps we are incorrectly assuming that early hominids' reproductive decisions were dictated entirely by male conflict. Female primates are just as sexually aggressive as their male counterparts, and likely play just as strong a role in the evolutionary development of mating strategies. In her quest to raise a high-investment, big-brained infant, which female wouldn't want a helping hand?
The evolutionary conundrum of human sexuality is complicated by the fact that some scientists dispute the three million year history of pair bonding. Judging from the lack of diversity in Y chromosome types, monogamy may not have emerged until the advent of agriculture, around 18,000 years ago.
Humans, of course, are notoriously bad at monogamy. Some of us never pair-bond, others pair-bond for only brief periods, others have multiple, concurrent, long-term sexual bonds. Many human societies are polygamous or promiscuous. Several believe in partible paternity, which hold that babies can have more than one father. Couple this diversity of mating with concealed ovulation and the fact that, for most of evolutionary history, we had no grasp of the link between sex and babies, and the picture of human sexuality becomes incredibly murky.
The fact remains that romantic pair-bonds and male child care are remarkable traits, at least compared to other great apes. They were no doubt watershed moments along the evolutionary path to modern Homo sapiens, even if it's still very unclear when or why they evolved.