Although polygamy is practiced in various cultures, humans still tend toward monogamy. But this was not always the norm among our ancestors. Other primates -- the mammalian group, to which humans belong -- are still polygamous, too.
"The modern monogamous culture has only been around for just 1,000 years," says Kit Opie, an evolutionary anthropologist from University College London.
Opie describes how the earliest primates -- as early as 75 million years ago -- were solitary and preferred to to live in isolation: "Adults would only come together to mate."
As time passed, primates as a whole became more social and evolved to live together in groups, but only humans became truly monogamous. Today, other primate species such as bonobos and chimps mate with multiple individuals in their groups.
"Humans shifted in the other direction," Opie said.
Why did it happen? Current theories suggest it's down to the preservation of an individual's health -- and their offspring.
STDs played a role?
As group sizes grew among human societies, from tens of people to hundreds of them, so may have the occurrence of sexually transmitted diseases, according to a recent study
Chris Bauch and his colleagues at the University of Waterloo in Canada used mathematical models to simulate the evolution of different mating norms in human societies. Using demographic and disease data, they found that when societies become larger, the prevalence of STDs becomes endemic (a regular occurrence) within the population. They suggest that this rise in STDs would have put social pressure on humans to stay monogamous in terms of their mating behavior.
"This research shows how events in natural systems, such as the spread of contagious diseases, can strongly influence the development of social norms and, in particular, our group-oriented judgments," Bauch, a professor of applied mathematics at Waterloo, said in a statement
The team suggests that in smaller societies, of 30 people or so -- typical of earlier hunter-gatherer populations -- STD outbreaks would have been short-lived and have had no significant impact on a population. However, as societies evolved and agriculture developed to make them even larger, rates of STDs would have been large enough that infertility from infections such as syphilis, chlamydia and gonorrhea would have been high, according to the research. Treatments for these conditions were not then available.
They suggest that monogamy would have therefore given males an advantage when producing offspring. The team also stresses that the STDs would have been a form of punishment for those who were polygamous.
Opie is not convinced of this theory, however, and believes that the larger societies stemming from the onset of agriculture and farming resulted in monogamy because people wanted to preserve their wealth through marriage.
"It's an interesting approach. ... You can imagine this maybe happening in larger societies," Opie said. "But it's marriage that matters here, as [this] is what passes on inheritance. ... Monogamy is a marriage system, not a mating system."
Bauch and his team noted that other factors would be involved, such as female choice. His team suggests that infections simply helped influence what is now a social norm. "Our social norms did not develop in complete isolation from what was happening in our natural environment."
Or is it all about fatherhood?
Opie does agree that larger group sizes -- and societies -- had a role to play in us becoming monogamous, but with a darker rationale: infanticide.
The team at UCL suggests that as primates developed and became more social, their brain size grew to accommodate this increased complexity over time. This in turn meant the brains of infants were larger than previous generations and required more attention -- and lactation -- from their mothers, resulting in females being less readily available to mate again after giving birth.
"Males [in the group] are basically sitting around waiting to mate with the female," Opie said. "It would therefore pay for the man to kill the infant, so he can mate with the female."
As the fathers would want their offspring to survive, they would nurture -- and protect -- them as necessary by pairing up.
In 2013, Opie published a paper arguing that monogamy came about so males could protect their infants. "One way to deal with this [risk of infanticide] is for the male and female to become a pair," he said.
Both theories remain exactly that -- theories -- without the options of a time machine and translator to go back to early human species and explore what happened to make us love the way we do today. But Opie also believes we're now slowly retreating away from this idea of staying with one partner.
"We're moving away from 'Until death do us part' as women are no longer willing to put up with [polygamy]," he said.
The future of that theory is ours to decide.