A fascinating new study suggests that the eldest generation of females has been responsible for humanity’s preference for monogamous mates over time. According to a report from Discovery News, the “evolution of grandmothering” has played a crucial role in many of the structures that define modern societies.

Professor Kristen Hawkes proposed the ‘grandmother hypothesis’ in 1997 based on observations of the Hazda people, a tribe of hunter-gatherers in Tanzania in the 1980’s. The recent study builds off of Hawke’s previous research.

Hawkes and her colleagues found that the older women in the Hazda tribe spent a significant amount of time caring for their grandchildren, making sure that they had enough food. In all primates except for humans, individuals fend for themselves once their mothers have finished nursing.

Hawkes suggested that when grandmothers contribute to making sure the youngest generation is well fed and taken care of, their daughters would be freed up to produce more offspring in a shorter amount of time.

The recent study used computer population models to demonstrate that the grandmothers with an inclination to help their daughters raise more children actually passed along genes that code for behavior that increases longevity, including sticking with a single mate.

The study shows how men and women interacted over a million-year time span. They found that over the course of 1 million years, grandmothering significantly increased the ratio of available males to females, maximizing the potential for population growth and thus survival.

This gave an advantage to people who could stick with a mate for extended periods of time. Those who devoted specific energy to maintaining the health of a family unit ultimately wind up having more children on average, thus solving the mystery of why humans are some of the only monogamous creatures on the planet.