How Women Got Their Menopause

The December 13 issue of New Scientist has an article about the evolution of menopause in humans [Are daughters-in-law to blame for the menopause? ]. The author is Alison Motluk, the Toronto corresponent for New Scientist.

She begins with ..

IT FLIES in the face of natural selection, yet in humans it seems fixed and universal: at around age 50, not far past the midpoint of life, normal healthy women lose their capacity to bear children. Following a decade of gentle winding down, the whole reproductive system screeches to a halt. It is as though, after a few years of wearing bifocals, all women suddenly went blind.

Menopause is a mystery. It leaves women with 20, 30, perhaps even 50 years of life - squandered time in evolutionary terms, because no further genes can be passed on. Yet the selection pressure for menopause must have been strong: there are no known pockets of women around the world who do not go through it. All the evidence suggests menopause has been around a long time, and that the age at which it hits has changed little. Increased longevity seems not to have budged our closing hours. Nor, apparently, has lifestyle; it hits hunter-gatherers at pretty much the same age as hip New Yorkers.

The search for an adaptive explanation for menopause has been going on for over fifty years.

The most common just-so story is called the "Grandmother Hypothesis." It imagines a time in the distant past when humans females were fertile until the day they died, which for 75% of women who survived childhood was before the age of 30 according to a recent study. A mutation arose in one individual and the effect was to induce menopause, or sterility, at about age 50. This new mutation proved to be so beneficial that it spread throughout the species. Today every woman undergoes the pain and frustration of menopause.

Why was it so beneficial? Because post-menopausal women whose partners were still alive could invest their time in looking after their grandchildren instead of having more children of their own.

To its credit, the New Scientist article reviews the latest work on the Grandmother Hypothesis and concludes, correctly, that the effect on ancient hunter-gather societies could not possibly have been significant enough to be adaptive.

The same reasoning applies to the "Mother Hypothesis," which claims that by going through menopause a mother will avoid the risks of future childbirths enabling her to concentrate on raising her existing children. Both of these hypotheses assume that "just saying no" was not a reasonable strategy in ancient societies and that's why menopause was necessary.

So what's the alternative if you are committed to an adaptive just-so story? You are going to be shocked ... menopause evolved to benefit daughters-in-law!!! (Our daughter-in-law will be so pleased.)

I'm not even going to dignify such a stupid idea by pointing out the obvious flaws.

In case you haven't clued in by now, the bottom line is that all these hypotheses are flawed from the get-go because they all make the unnecessary assumption that the pain and suffering of menopause are adaptive. What if they aren't? What if menopause is neutral with respect to evolution or even maladaptive? What's the point of making up irrational just-so stories when there's no evidence to suggest that menopause has any positive effect of fitness?

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