From Science:
Most women have their last child before age 40. Why would Darwinian evolution favor such a cutoff, especially when most other mammals reproduce until they die? A new study finds support for the “grandmother hypothesis,” the idea that older women spread their genes most effectively by helping their daughters take care of their children. In 1998, behavioral ecologist Kristen Hawkes of the University of Utah in Salt Lake City and her colleagues proposed that grandmothers lend their skill and experience to the rearing of their grandchildren. Hawkes and others cited the Hadza, a modern foraging society in Tanzania, in which grandmothers search for tubers while their daughters are breastfeeding their babies. Given that tubers are thought to have become an important staple during the early days of human evolution, a selective advantage for “grandmothering” rather than “mothering” by older women might have arisen in our species.
Over the past decade, a number of researchers have tried to test the hypothesis by looking at the relationship between grandmothers and their grandchildren. Some studies found that when grandmothers live near their grandchildren and/or live longer, their grandchildren have higher survival rates. But other studies did not see this correlation.
More here.
