A new finding published in the Proceedings of the National Academy of Science says that grandmas are to thank for monogamous mates.

The ‘grandmother hypothesis’ was proposed in 1997 by Professor Kristen Hawkes who recently co-authored the newest study. Her original findings were based on her observations in the 1980’s of the Hazda hunger-gatherer people in Tanzania, according to Discovery News.

In Hawkes’s study, it was noted that older women of the tribe spent most of their days collecting food for their grandchildren in comparison to all other primates and mammals that gather their own food after weaning.

She then proposed that grandmothers helped to feed their grandchildren after weaning so that their daughters could go on and produce more children at shorter intervals. Hawkes, from the Department of Anthropology at the University of Utah, used a computer model to display that by allowing their daughters to have more children, those ancestral females lived long enough to become grandmothers passed their longevity genes on to more descendants who then had longer adult lifespans.

The simulations showed from a start point that just 1 percent of women living all the way to grandmothering age withing 24,000-60,000 years about 43 percent of adult women are grandmothers. This figure is consistent with today’s hunger-gatherer populations.

In the more recent study, Hawkes examined how evolution of grandmotherting impacted male to female interaction. They compared simulations of male to female sex ratios of great apes, with no grandmothering, to moder-day human hunter-gatherer populations.

When ‘grandmothering’ was present, they found that over a million years, the ratio of available males to females ready to conceive doubled, averaging about 111 males for every female. The co-author of the study, Dr. Peter Kim at the University of Sydney, said that the increase in human life span meant that while older women were becoming more fertile after menopause, older men remained able to reproduce.

Over a long period of time, this created an imbalance in the sex ratio of fertile adults increasing competition among men for the females that were still fertile. Therefore, the increased competition led to reduced success in finding a mate, but it made it more important to find a mate to guard them.

Kim said that this would answer the mystery of, “why humans are such highly monogamous species while other species are not”.

“Grandmothering first drove the change in human life histories and that caused there to be a shift in fertile male to fertile female ratios and that caused the next shift,” Kim says.

“Suddenly males optimal behavior is to link with one female and to stay and protect them and have all their kids with that female.”