A couple of years ago I saw some touchy-feely article by a feminist arguing that grandmothers played a major role in the development of human society--ignoring the fact that most young mothers would not have had a living mother, and the elephant in the room: the prevalence of monogamous couples, where the mother has a husband to help her survive.
Probably the same article that I read or was referenced. It seemed plausible. As mentioned in this article is probably only recent.
It was all about survival and if the elderly were more of a burden than a benefit, they were expected to wander off and die.
I can certainly see how useful and helpful a grandmother might be... as they can be today.