Evolutionary significance of grandmothers

From this Scientific American article:

The evolutionary biologist [Virpi Lummaa of the University of Sheffield] has also used this historical data set to ponder the conundrum of grandmothers. That is, why human women often live long after they are able to reproduce (on average around the age of 50), unlike almost all other animals. "If your ultimate purpose in life was to create as many offspring as possible or pass off as many genes," Lummaa says, "it's kind of strange that human women stop halfway."

One possible explanation is that having a grandmother around somehow improves the reproductive potential of her grandchildren. In fact, that is exactly what the researchers found when they reviewed stats on 537 Finnish women who had a combined total of 6,002 grandchildren. Adding in data from more than 3,000 French Canadians (who had a modest 100,074 grandchildren) confirmed that having grandma around to help enabled younger women to have more children sooner and with improved chances of surviving into adulthood. "That suggests that perhaps one reason why women do carry on living is because they are able to help," Lummaa says.

How about grandfathers?

If grandmothers improve survival odds, what do elderly males contribute? "If anything there's a negative effect," she says. This could be because of the cultural tradition of catering to men, particularly old men. "Maybe if you had an old grandpa, he was eating your food," she speculates. Or it could be that because men can continue to reproduce, they are less vested in anyone other than their own children. Another possible reason is that women can be sure that a grandchild is their genetic descendant, but it is more difficult for grandfathers. This may also have spurred them to seek second and even third wives rather than focusing on their children. "We are comparing men who married once in their lifetime[s] with men who are married several times," Lummaa says.