Humans evolved to being monogamous to protect their children from being killed by other males and this ultimately led to more intelligent offspring, say scientists.
A team from London, Manchester, Oxford and Auckland studied the mating habits of monkeys and discovered that mothers will delay mating when nurturing young children.
In non-monogamous communities, rival males will try to kill children to encourage the female to want to mate sooner. However, in societies where primates choose a mate and stay with them, the males are more likely to care for their offspring and want to protect them, and this leads to more intelligent, well nurtured children.
The scientists believe this provides ‘conclusive proof’ that protecting young children is the main reason for monogamy in humans.
The team from University College London worked with researchers from universities in Manchester, Oxford and Auckland to gather data across 230 primate species.
They created a family tree and re-ran evolution millions of times across it to discover whether different behaviours evolved together across time, and if so, which behaviour evolved first.
This allowed them to determine the timing of ‘trait evolution.’
They discovered that high levels of male primates killing offspring led the groups to switch from a multi-male mating system to a monogamous one.
Then, when the monogamous community was established, the fathers and mothers began sharing the care duties – rather than women being left to nurture their young themselves.
Following the emergence of monogamy, males are then more likely to care for their offspring.
By staying together, the male offers protection against this happening by guarding his child.
He can also share the burden of childcare and this may have led to the development of our complex, intelligent brains, the study shows.
It is the first research to reveal this evolutionary pathway, which the authors say they have ‘conclusively’ proved.
UCL’s Dr Kit Opie, lead author of the study, wrote in journal Proceedings of the National Academy of Sciences: ‘This is the first time that the theories for the evolution of monogamy have been systematically tested, conclusively showing that infanticide is the driver of monogamy.
‘This brings to a close the long running debate about the origin of monogamy in primates.’
An additional benefit of sharing the burden of care is that females can then have more ‘costly’ young.
The authors say that the price of living in complex societies has resulted in many primate species having large, and costly, brains.
In evolutionary terms, growing a big brain is expensive and requires that offspring mature slowly.
Caring fathers make this easier by helping out with young with long childhoods and this may explain how large brains could evolve in humans.
Dr Susanne Shultz, from the University of Manchester, said: ‘What makes this study so exciting is that it allows us to peer back into our evolutionary past to understand the factors that were important in making us human.
‘Once fathers decide to stick around and care for young, mothers can then change their reproductive decisions and have more, brainy offspring.’
