grandparents

For the past 55 years, scientists have theorized that a major reason why humans live so much longer past their reproductive years than other species is because of grandmothers.

The "grandmother effect," as it's known, postulates that in hunter-gatherer societies, grandmothers played a vital role in finding food and raising children. In fact, the grandmother's role was so important that it had a huge impact on whether or not children survived.

"By relieving a mother of some of her child-raising responsibilities, so the thinking goes, grandmothers make it easier for their daughters to have more children and also make it possible for those children to have longer lives by helping them during the difficult early years of life," Haider J. Warraich writes in Stat.

Two studies further this hypothesis by showing the important roles that grandmothers have in the survival of their grandchildren. A study of birth and death records in Finland for individuals born between 1731 and 1890 found that having a maternal grandmother between the ages of 50 and 75 increased a child's survival rate.

Another study found that proximity to grandmother matters, too. The shorter the distance between grandmother and grandchild, the more involved the grandmother can be and the more benefits that accrue to her daughter and grandchildren.