We now know that the first hominins, which emerged more than seven million years ago, might have been monogamous. Humans stayed (mostly) monogamous for good reason: it helped them evolve into the big-brained world conquerors they are today.
According to the New York Times, a 2011 paper showed that early humans, or hominids, began shifting towards monogamy about 3.5 million years ago—though the species never evolved to be 100% monogamous (remember that earlier statistic).
The violent sexual anarchy of the early human horde gave way to a still-violent but at least monogamous savage life, which gradually led to the genteel courtships of their own day.
Our prehistoric male ancestors kept female harems and fought over them to procreate: because the potential number of offspring was greater for males, competition for mates was severe. As a result, evolutionary forces focused on making males big and strong, rather than long lived.
Most hunter-gatherers were monogamous. Most hunters could provide only enough meat for one wife and her children. The best hunters could support two wives (polygyny).
In the Middle Ages, as in other ages, powerful men married monogamously, but mated polygynously. Both laymen and church men tended to have sexual access to as many women as they could afford. But first-born sons were allowed a legitimate wife, on whom they got legitimate heirs.
It's thought that at one time, human ancestors did engage in chimp-like habits of sex and child-rearing, in which strong alpha males mated freely with the females of their choice, and then left the child-raising duties to them.
We now know that the first hominins, which emerged more than seven million years ago, might have been monogamous. Humans stayed (mostly) monogamous for good reason: it helped them evolve into the big-brained world conquerors they are today.
Early humans and other hominins such as Neanderthals appear to have lived in small family units. The small population size made inbreeding likely, but among anatomically modern humans it eventually ceased to be commonplace; when this happened, however, is unclear.
Not many species are strictly monogamous, people might be more polygamous than you would think. Humans aren't sexually monogamous in the sense that many birds are. Geese form lifelong couples and virtually never mate with anyone except their partner.
As some of the first bands of modern humans moved out of Africa, they met and mated with Neandertals about 100,000 years ago—perhaps in the fertile Nile Valley, along the coastal hills of the Middle East, or in the once-verdant Arabian Peninsula.
The Neanderthals had long ring fingers, suggesting they were a promiscuous bunch — like many primates alive today they probably lived in groups. Males may have likely either kept harems of female mates, or males and females each mated with multiple partners.
The average age at menarche for modern hunter-gatherers seems a much more accurate estimation for a Paleolithic woman). This means that the average woman would have Child 1 at 19, Child 2 at 22, and Child 3 at 25 – and then, according to the “cavemen died young” theory, she would die.
Ancient Greeks
Like many Western societies, the ancient Greeks and Romans were monogamous on paper (men could not marry more than one woman, for instance, nor could they live with their concubines), but not so much in practice, particularly if you were a man.
As Christianity emerged in the Roman Empire in the first centuries AD, it embraced monogamy and took it further, insisting that two people must reserve their bodies and desires for each other, marriage becoming 'an everlasting threesome with God'.
Evolution dictates that genes have the final say. And if there is one thing genes want, it is to spread as far and wide as possible. That is why monogamy is rare among mammals. Females have to wait for a long gestation period to have a child, where as males could go and inseminate many other females in that time.
The sordid truth is out, and its not what scientists expected. The closest-ever look at the Neanderthal genome reveals that yes, we did interbreed.
Band of (Inbred) Brothers
Early humans most likely lived in small groups spread far from each other, and the geographical limitations of their lives would have made reproducing with relatives inevitable.
Denisovans are close relatives of both modern humans and Neanderthals, and likely diverged from these lineages around 300,000 to 400,000 years ago; they are more closely related to Neanderthals than to modern humans.
For humans, monogamy is not biologically ordained. According to evolutionary psychologist David M. Buss of the University of Texas at Austin, humans are in general innately inclined toward nonmonogamy.
Practically, the justification for being in a polygamous relationship differs for men and women. Most men do so, because of their desire to have a larger number of offspring and most effectively increase their fitness by having many sexual partners.
Only about 2% of the global population lives in polygamous households, and in the vast majority of countries, that share is under 0.5%.
Humans are pretty unusual in having sex throughout the year rather than saving it for a specific mating season. Most animals time their reproductive season so that young are born or hatch when there is more food available and the weather isn't so harsh.
Men were sometimes able to choose their bride. Marriage back then was not based on love; most marriages were political arrangements. Husbands and wives were generally strangers until they first met. If love was involved at all, it came after the couple had been married.
Among the aristocracy in the early Middle Ages there are occasional references that suggest girls might marry in their mid teens. The legal age for marriage set by canon law was twelve for girls and fourteen for boys.