Well, a few writers have thunk it, anyroad! Jared Diamond springs to mind and Elaine Morgan. I'm sure other people can mention more. The point is that grandparents help with childcare and with their wisdom acquired over the years. From a purely genetic and survival point of view, after parents, grandparents are the people most closely related to their grandchildren and so also the most likely, in biological terms, to help ensure their grandchildren's survival. Evolutionary psychology, that's the term I was searching my brain for. There's an interesting, and thankfully slim, volume called The Truth About Cinderella on this very subject by Martin Daly and Margot Wilson.