This sweet painting by Ferdinand Georg Waldmüller from 1854 is called _Grandmother with Three Grandchildren_. In it , three cheery young girls surround a seated older woman, eyes gazing on some scrap of paper she is holding. All four females are comfortable with each other, and wear relaxed and happy expressions. The image of sweet but physically weak grandmothers might be typical, but it belies the invaluable contributions they have played in human survival.

Research by Kristen Hawkes and others in the last decades has shed light on what has come to be known as “the grandmother thesis” in evolutionary studies. Looking at data from non-human primates, statistical modeling, and modern grandmothers in various communities, the thesis runs like this:.

By providing supportive child care to their children, grandmothers enabled women to bear infants at shorter intervals and to ensure the healthier survival of their children’s toddlers and young grandchildren. Mothers historically have to use an enormous amount of their energies on their infants, but grandmothers could provide safety and food to the other youngsters, which enabled the greatest number of children to survive.

In fact, we might owe grandmothers even more than this. The critical value of grandmothers to their descendants might be the cause of humanity’s relative longevity — we outlive most other mammals. Human females reach menopause relatively early in their lifespans, but because grandmothers’ longevity would have helped her grandchildren to survive, natural selection over time would have favored longer-living women, despite the fact that they were not reproducing.

Much of this research got going with analysis of grandmothers in the modern hunter-gathering peoples of the Hazda in northern Tanzania. Kristen Hawkes documented the larger and healthier families there who had grandmothers to help provide food and childcare.