Life After Fertility
Anthropologists and evolutionary biologists have wondered for more than 50 years about the purpose of menopause. Now, some new studies shed light on the subject and also attempt to answer the question of why modern women outlive their fertile years by such a long span of time.
According to the theory of evolution, once a woman passes her fertile years, she has outlived her purpose for living. Once women can no longer breed, the selection of genes, complicit in evolution and in the survival of the fittest, can no longer take place. A woman's ovaries cease to produce eggs when she enters her 50's at which point, evolutionary theory would have us believe she has come to the end of her reason for existence.
One theory, known as the "grandmother hypothesis" explains the seeming conundrum of the long postmenopausal years by claiming the necessity of a grandmother to ensure the survival of the grandchildren, so that her own genes will flourish through this generation. Those skeptical of this theory believe that this theory goes wrong when one applies basic math to the situation. A woman who bears children passes down half of her genes, but a grandmother passes only a quarter of her grandmotherly genes. Why should a woman give up bearing her own children to assist in the continuation of a smaller number of her own genes?
Evolutionary biologist Michael Cant, of the University of Exeter in England, and co-author of a study on menopause says, "The problem is that these grandmother benefits aren't big enough to ever favor stopping breeding between the ages of 40 and 50. When you look at data from hunter-gatherers and other natural fertility populations, the sums just don't add up." Cant feels that grandmothers may benefit their grandchildren in many ways, but that the genetic tradeoff just isn't that great when compared to bearing another child.
Cant and his co-author, evolutionary biologist Rufus Johnstone, from the University of Cambridge, employed game theory to demonstrate that menopause evolved because of a conflict between reproducing generations. In certain species, young females don't begin to reproduce until they've assisted the older generation in bearing females. In human society, the younger women supplant their elders and force the older women to step down from their breeding tasks. Cant remarks that primates also have a postmenopausal life span, but humans are remarkable in that there is no overlap between the breeding generations.
Anthropologist Kirsten Hawkes, of the University of Utah, birthed the grandmother hypothesis but says that Cant and Johnstone are correct in focusing on the conflict between the generations. Hawkes mentions whales giving birth in their 80's and elephants bearing well into their 60's and posits, "It's clearly something selection can adjust. So explaining why it hasn't in us has to be part of the story."
Another theory, the "mother hypothesis," explains that older mothers have more to gain by devoting their resources to the children they already have, rather than bearing new children. This seems to be borne out by the fact that the infants of older women are at a greater risk for birth defects and stillbirth, and the older mother herself is at an increased risk for dying in childbirth.
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