The Grandmother Hypothesis and Alloparental Care
The grandmother hypothesis suggests that the long post-reproductive life span of human females evolved as a fitness benefit. That is, if women gained an extra fitness benefit by investing in their adult offspring and their grandchildren rather than reproducing until old age, selection could have favored a longer life span. The hypothesis is supported by many recent empirical studies that show the effect of grandmothers on a wide range of life-history traits in different settings worldwide, including Africa and Asia.
The Role of Grandmothers in Childcare
One of the most important things that grandmothers do is provide alloparental care to their daughters' children and their grandchildren. This includes everything from feeding the young to playing with them. In other words, they take care of the kids so that mothers can focus on the birth and growth of their own children. It's a form of social support that increases the chance that the child will survive to adulthood and leave behind more copies of her genetic code in subsequent generations.
Implications for Human Lifespan and Brain Size
This theory has been supported by a growing body of scientific evidence that shows that human life spans are higher than those of other animals. It also explains why human brains are larger than those of other primates. Hawkes argues that this is because human grandmothers have had an important role in the evolution of the post-reproductive life span of their daughters and their grandchildren, increasing the chances of survival and leaving behind more of her DNA. She and other researchers have found this to be true in a variety of settings, from the Kalahari Desert to the Cook Islands.
The Influence of Proximity
They also find that this effect is strongest when the grandmother lives closer to her granddaughters, making it easier for them to provide this help. That means that in pre-industrial societies, where women lived long past their reproductive years, the influence of grandmothers was especially strong.
Support and Uncertainties
Despite the growing support for this idea, it remains unclear whether it's an actual evolutionary mechanism or just a hypothesis that's based on observational data. But if it is, it could help explain why humans seem to have had such long lifespans until recently.
Study on Historical French Settlers
The earliest French settlers in what is now Quebec had a lot of grandmothers, so Bergeron and his colleagues took advantage of the data to look at how these grandmothers affected their daughters' life-history traits. They looked at a large dataset of parish records that tracked every birth, death, and marriage in those communities from the 16th to 18th centuries.When the researchers compared a daughter's number of children and their longevity with that of her sister, they discovered that daughters who had living mothers had, on average, about two more children than those whose mothers had died. Those daughters were also more likely to have a child who survived to 15 years of age.In a follow-up study, researchers explored how the distance between a mother and her children might have affected this effect, finding that the closer a woman was to her daughter, the more likely she was to be able to help her daughter with raising her children. They studied a large database of historical French settlers in the St. Lawrence River valley, a location that was both urban and rural. They combed through the data to see how far, in kilometers, a mother and her daughter were from each other.
