Judith Shulevitz at The New Republic homes in on two theories: the “stopping-early hypothesis” and the “grandmother hypothesis”:
In 1957, the evolutionary biologist George Williams proposed what is called the “stopping-early” hypothesis: Middle-age women need baby-free time to usher their youngest children into adulthood. In the 1980s, an American anthropologist named Kristen Hawkes and two colleagues came up with a different explanation. They had gone to northern Tanzania to study the foraging habits of the Hadza, the last known hunter-gatherers in Africa. While there, the scholars were struck by how strong the tribe’s old women were and how, rather than live off the fruits of others’ labor, they worked hard digging up the tribe’s main starch staple, a deeply-buried tuber. “Their acquisition rates were similar to the rates of younger women,” Hawkes told me, “but these old ladies were spending even more time” than their daughters gathering food, leaving camp earlier, coming back later, and bringing back more than they needed. The anthropologists also noticed that many children with grandmothers or great-aunts had faster growth rates than their counterparts.
From these slim clues, Hawkes and her colleagues developed the “grandmother hypothesis,” which holds that women past childbearing age helped not just their children, but their children’s children, and lengthened the human lifespan in the process. Without babies of their own to lug around, grandmothers had both time and a very good reason to be useful. When they eked out food for their daughters’ children, they reduced the chance that those children would die. That gave the grandmothers a better chance of passing on their own predisposition to longevity. [...]
Two decades later, the grandmother hypothesis has gone from oddball conjecture to one of the dominant theories of why we live so long, breed so fast, and are so smart. The extra calories and care supplied by women in their long post-fertile period subsidized the long pre-fertile period that is childhood. And that’s what made us fully human.
I’ll add a few guesses to these hypotheses (which is what we’re basically doing absent hard evidence):
- Group selection. If group selection plays a role in human evolution–and there are evolutionary biologists like David Sloan Wilson who argue that it does, then giving care to grandmas (and not just exploiting their labor) adds dignity and value to the tribe as a whole. It says: Healthy or unhealthy, we don’t leave our wounded and elderly on the battlefield of life to die. We are all ‘in’ with one another. We rise and fall together. This bonding gesture between young and old makes every individual more willing to take risks on behalf of the others in the tribe, and to expect similar gestures in return. This could certainly be an evolved group trait that raises the fitness of the group as a whole.
- Grandmas raise courage. The existence of grandmas may add spine to warriors. Men fight for the women and children back at the home base, not for themselves. This gives men a righteous cause born of love (for mothers, for mates, for children) and probably makes them more effective and less self-absorbed fighters.
- Grandmas inspire love. Grandmas may have brought selective pressure to the evolution of love. We love our mothers and grandmothers. Their bond with us runs deep. And the emotion of love makes for a stronger collective group. When you love somebody so much that you will not let them die or be abandoned no matter what, you demonstrate traits that work in more adaptive contexts as well. (Who wants a mate that dishonors and abandons his or her parents? If you’re in the mating game, you want to have a reputation for loving your parents.)
- Grandmas carry cultural memory and know-how. This one seems almost too obvious to point out.
These guesses broadly fit within the “grandma hypothesis” generally. The extension of the lives of post-menopausal women have all sorts of benefits to group survival (some obvious, others less so).
If anybody can think of other evolutionary advantages conferred from extending the lives of grandmas (and grandpas as well), I’d be curious to know your thoughts. It’s an important issue going forward, obviously. Social policy in the United States seems to be favoring the idea that the elderly are, on balance, a burden. And we worship youth and fear growing old.