Humans - and our ancestors - have ruddy large brains. This has given us many evolutionary advantages, but also stuck us with some big disadvantages. Some noisy, messy, full-diapered disadvantages: children. As such, monogamy may have been just as important to our evolution as our brains.
At least, that's what the monogamy hypothesis claims. It notes that our big brains mean our kids are more dependent on their parents for a longer period of time. They take longer to mature, are really helpless, and need to be taught a lot. As such they need extra childcare when compared to our ape brethren.
Might monogamy - which involves both parents helping their children - have been the key adaptation that made this possible.
The monogamy hypothesis
The principles of the monogamy hypothesis stem from research on other animals that engage in a lot of childcare. Researchers noticed that these disparate - but caring - species often shared several interesting features.
One such similarity was the fact members of the family apart from the mother helped with child care. Siblings or the father were common babysitters; helping give the offspring the increased childcare they needed. Another interesting similarity was that these species were often very good at recognising kin; partly so they could help them, but also to stop any incest occurring. And as Game of Thrones has shown, incest rarely leads to anything good. Notably, almost all of these characteristics are also shared by humans.
But perhaps the most important feature seen in all these animals is monogamy. Or, rather, it wasn't seen in all the species but it was persistent enough that it appeared to be the ancestral form (or at least, much more likely to be its ancestral form). So whilst modern co-operative breeders can practice polyandry, polygyny, or just about anything else; a monogamous lifestyle was so common that it seems to have been the starting point for all these species. The later variants arose once the co-operative breeding was well underway and thus being totally faithful wasn't as necessary.
Given these many similarities between all these animals, and humans, it was thought that our ancestors may have followed a similar pattern. Although we now practice many different forms of breeding; we were originally monogamous. This helped the development of co-operative breeding so we could care for our tiny, vulnerable babies. Once that was sorted out, monogamy became a bit more optional; as long as childcare remained taken care of.
Fossils in love
However, monogamy and co-operative breeding aren't always linked in those other animals. Might humans be one of the rare exceptions? After all, upwards of 80% of modern groups include more than two people in a marriage.
To examine this possibility other researchers began looking for possible biological indicators of our past relationships. After all, if these relationships influenced our biology they must have been around long enough for evolution to be influenced by them (or vice versa). Interestingly, they did find some such features.
For example, human males have small testicles. At least, relative to chimps and our other ape relatives. In the animal kingdom, the larger the sack the more promiscuous the society. The extra sperm is needed to outcompete all the other males a potential partner has likely been sleeping with. We also have reduced sexual dimorphism. Large males are important for competition of the non-sperm kind; to help them beat other males and mate with a lot of women.
Interestingly, this pattern of reduced sexual dimorphism seems to be ancient. Large male canines are a sign of such competition, allowing men to fight and bite each other for mates. Although the earliest hominin males seem to have had large canines (even canines that could self-sharpen for added damage); by the time of Australopithecus this had vanished. Teeth were similar across genders, suggesting there was already reduced competition for women.
Meh for monogamy
Although this biological evidence might seem to seal the deal for monogamy, it doesn't quite have the bite of a pre- Australopithecus hominin. Notably, none of it is directly correlated with monogamy; but with features often associated with this behaviour. Like the testicles, which indicate reduced sperm competition (and thus promiscuity). Monogamous relationships don't necessarily rule out promiscuity; as many a reality TV show will reveal. And if you're looking for a slightly more scientific source; check out the superb wren-fairy (a bird with a superb name). Although they're one of the most faithful birds in the animal kingdom, they also have one of the highest rates of promiscuity of any bird.
Thus; we can't say that humans were initially monogamous. However, their dependent kids mean that childcare was very important. To the point where assistance - be it from the father, grandparents or siblings - would have been necessary. Whether or not it was monogamy that motivated others to stick around and help is a different question.
For all we know, it might be a recent development.
Summary
Although there's some evidence for an ancient origin of monogamy, the jury is still out. One thing we can say is that our big brains meant co-operative breeding was definitely important.
References
Cornwallis, C.K., West, S.A., Davis, K.E. and Griffin, A.S., 2010. Promiscuity and the evolutionary transition to complex societies. Nature, 466(7309), pp.969-972.
Kramer, K.L. and Russell, A.F., 2015. Was monogamy a key step on the Hominin road? Reevaluating the monogamy hypothesis in the evolution of cooperative breeding. Evolutionary Anthropology: Issues, News, and Reviews, 24(2), pp.73-83.
