Jonathan Lambert on the theory for why, almost alone among species, human females live on well past their reproductive years:
Killer whales, Japanese aphids and Homo sapiens — they’re among the few organisms whose females live on long past the age of reproduction.
Since the name of the evolutionary game is survival and reproduction, the phenomenon begs explanation — why live longer than you can reproduce? In the 1960s, researchers came up with the “grandmother hypothesis” to explain the human side of things. The hypothesis is that the help of grandmothers enables mothers to have more children. So women who had the genetic makeup for longer living would ultimately have more grandchildren carrying their longevity genes. (Sorry, grandfathers, you’re not included in this picture.)
Two studies published Thursday in Current Biology take another look at this hypothesis and add new insights into the role grandmothers play.
The first hard evidence for the grandmother hypothesis was gathered by Kristen Hawkes, an anthropologist at the University of Utah who was studying the Hadza people, a group of hunter-gatherers in northern Tanzania. Hawkes was struck by “how productive these old ladies were” at foraging for food, and she later documented how their help allowed mothers to have more children.
If our long post-reproductive lives evolved because of grandmothers, we should be able to find fingerprints of the benefits of grandmothering in many cultures. But the circumstances of modern life differ drastically from those we faced at the beginning of our evolutionary story.
The studies in Current Biology turned to the detailed records of two preindustrial populations, one in what is now Quebec and the other in Finland. The researchers mined these rich databases to quantify the reproductive boost that grandmothers provide and to help us better understand the limits of their help.
H/T to Claire Lehmann for the link.
