Women should not only be thankful to the grandmothers for our increased longevity, but should also thank them for helping men to turn into monogamous partners. This has been suggested by a new study published in the popular journal Proceedings of the National Academy of Science (PNAS). The findings of the said study build on earlier research on the evolution of grandmothering.

Prof. Kristen Hawkes, who is one of the coauthors of this new study, was the first to propose the “grandmother hypothesis”. In 1997, Prof. Hawkes and her colleagues first talked about the grandmother hypothesis after observing Tanzania’s Hazda hunter-gatherer tribe during the 1980s.

The research team under Prof. Hawkes found that older women in the tribe used to collect foods for their grandchildren. Here, it must be mentioned that all mammals and primates except the humans start gathering their own foods after weaning.

Prof. Hawkes, who currently represents University of Utah’s Department of Anthropology, proposed that as these older women fed their grandchildren after they stopped weaning, the mothers could give birth to more babies at smaller intervals.

Then, using computer modeling, the research team showed that by giving the mothers the chance of giving birth to more babies, the elderly women, who had enough longevity for becoming grandmothers, ensured that their longevity gene is passed to greater number of descendants. As a result, those descendants automatically got much longer adult life spans.

For the new study published in PNAS, Prof. Hawkes and her colleagues again took help of computer modeling. This time the technology was used for examining what kind of impact evolution of grandmothering had on female to male interactions.

The researchers compared the simulations of female to male sex ratios of the great apes, creatures that never practiced grandmothering, with the modern-day hunter-gathers tribes. It was found that the ratio of males available to ready to conceive females doubled in the presence of grandmothering. According to the researchers, in the presence of grandmothering, every female on an average had 111 males for her.

According to University of Sydney’s Dr. Peter Kim, a coauthor of the study, the findings of this study explains why the humans are mostly monogamous although all other species living around them are not. It can be said that this study links the optimal behavior of males with one female.