At a recent international conference — the first devoted to grandmothers — researchers concluded with something approaching a consensus that grandmothers in particular, and elder female kin in general, have been an underrated source of power and sway in our evolutionary heritage. Grandmothers, they said, are in a distinctive evolutionary category. They are no longer reproductively active themselves, as older males may struggle to be, but they often have many hale years ahead of them; and as the existence of substantial proportions of older adults among even the most "primitive" cultures indicates, such durability is nothing new.
Having a mother's mother around significantly improves the survival prospect of an infant, for example, and in some cases this is especially or only true for boys. (Paternal grandmothers don't confer the same benefit, interestingly enough.)
Anyway, the New York Times article is an interesting and provocative read.