Murder is the main reason why humans and other primates mate for life, according to scientists.
Infanticide was the key driving force that caused us to evolve into a monogamous species, it is claimed.
Males of some animals, including lions and brown bears, kill the young of unrelated females to improve mating opportunities.
The practice arises when females nursing slowly developing and vulnerable young are forced to delay further conception.
Monogamy both provides extra protection for the infant and, by sharing the burden of care, shortens the period of infant dependency.
Females are then able to reproduce more quickly, and can afford to have more costly young that mature slowly.
A long childhood appears to be necessary for growing a large brain, making monogamy distinctly advantageous to humans.
It could explain why, uniquely among primates, humans have both a very long childhood and mothers who reproduce quickly.
Scientists explored the evolutionary pathway that led to human monogamy by gathering data from 230 primate species.
The information was used to construct a family tree of inter-species relationships.
Analysis of evolving traits revealed that male infanticide was the chief reason for the switch from a multi-male mating system to monogamy.
The findings are published in the journal Proceedings of the National Academy of Sciences.
Lead researcher Dr Kit Opie, from University College London, said: "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."
Colleague 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."