Why did humans become monogamous, apparently rejecting the promiscuity that is natural to most animals?
Was it morality? Religion? Maybe love?
The answer is germs, researchers said Tuesday, arguing that the havoc caused by sexually transmitted diseases (STIs) convinced our ancestors it would be better to mate for life.
A research duo from Canada and Germany observed that STIs flourished among large groups of people living in the villages, towns and cities that arose after prehistoric hunter-gatherers settled down to farm.
Left unchecked, spreading diseases can affect individual fertility and a group's overall reproduction rate.
Falling population numbers would force a rethink of sexual behavior - which in turn gives rise to social mores.
The researchers developed a mathematical model of hunter-gatherer demographics and likely STI spread among them.
They used it "to show how growing STI disease burden in larger residential group sizes can foster the emergence of socially imposed monogamy in human mating."
In small groups of no more than 30 individuals, with no chance for epidemic spread, STI outbreaks are generally short-lived, the team said.
The reduced risk may explain why small groups, both among early humans and today, are often polygynous (when men have more than one partner).
Socially-imposed human monogamy has long been considered an "evolutionary puzzle," according to the research duo.
It requires societies to put in place checks and structures - a police and court system, for example - to uphold societal mores.
"Yet, many larger human societies transitioned from polygyny to socially imposed monogamy beginning with the advent of agriculture and larger residential groups," said the paper.
That riddle may now be solved.
The research showed that our natural environment, with factors such as disease spread, "can strongly influence the development of social norms, and in particular our group-oriented judgements," study author Chris Bauch of the University of Waterloo in Canada told AFP.
But this did not necessarily mean that humans would become wildly promiscuous if drugs were to make STIs a thing of the past, he added.
"Modern societies are more complicated... and there is probably more than one reason that explains socially imposed monogamy," Bauch said by email.
"I think it is premature to speculate that marriage will disappear, or that polygyny will return, if we solve the problem of STIs."
The research was published in the journal Nature Communications.