Life History, Genetic Relatedness, and the Evolution of Menopause
Imagine you’re on the Serengeti Plateau and your children are hungry. For miles in every direction there’s nothing but dry scrub grass with the occasional flat-topped acacia tree marking the landscape. Your oldest has found a spot to dig for tubers but he and your daughter aren’t strong enough to scrape away the hard, baked earth by themselves. Your husband is tracking a wounded gazelle and could be gone for days. Meanwhile, the infant slung to your hip has started screaming and the distinctive sound triggers a release of oxytocin that causes your breasts to swell and leak. You have to feed her but you can’t do that and make sure your other children get enough to eat. There’s a very real chance that some of them will be too weak to survive the next time fever breaks out unless you can get help.
Read this excellent review here.
That’s a guest post on Carin Bondar’s blog written by Eric Michael Johnson. If you like the post you can vote for it in the Quark Award … Just go here and search for ‘Dr. Carin Bondar” and vote for that article (which is actually by Eric Michael Johnson)
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And for their next trick, they will imagine some plausible- sounding reason why men do NOT become infertile at age 45. Of course, we will assume from the start that males’ continuing fertility is also an adaptation. Because anything we observe must be an adaptation, right?
Well, you are going have to work a little harder than that to convince me that the ability to produce gametes in a long lived multiparous species is not tied into the whole adaptation thing. And, actually, cessation of gamete production that is not simply running out because of unexpected lifespan extension does look like something. Personlly, I’m totally down with the grandmother hypothesis. It is nothing like a “just so” story.
Here’s a small twist: the article mentions that female chimpanzees also reach a reproductive senescence. They just die about the same time. Perhaps the adaptations aren’t for a longer period of female infertility specifically, but for a longer human lifespan. If the changes that boost longevity don’t also increase the period of female fertility, that produces a longer period of infertility, just as a matter of arithmetic.
Russell, that is certainly a possibility. However, assume that the forager lifestyle used today to calculate these numbers applies to the period of human evolution over which such a change would have occurred. Wrangham and I postulated that a shift to a root eating diet would have been quite early in human evolution (and this is one of the ways the hypothesis has been calibrated and supported) possibly at the chimp-human split. So we’ve had a period of five million years or so of using roots as fallback foods, and some time during that period, maybe two million years ago or a bit less with H. erectus, lifespan increases enough for many females that reach “middle age” to live into what is now menopause.
Simple math based on lifespan of chimps might suggest that an extended lifespan simply goes beyond reproductive age, but math almost as simple having to do with fitness strongly suggests that if lifespan is extended reliably past the usual age of fertility, that there would be selection to extend the period of fecundity.
Extending fecundity is probably not that hard. All one has to do is slow down the rate of ovulation and hype up protection (of ova) mechanisms a bit. Hominoids (apes, etc.) are already long lived. Extending the period of potential fertility is not at all out of the question.
Yeah, that makes sense. Though it reminds me how much data we’re missing. We only know when reproductive senescence occurs for current populations, human and our closest relatives. Not having that through time, it’s hard to know how it has changed through the various lineages. At some point, I suspect genomics will unravel some of this.
We know from bones and teeth that pre 2mya australopiths had an ape-like developmental pattern, and after, a human-like one in some resepcects becoming more human like over time . Not scenecence specifically, but these things tend to be linked.
As an interested layperson and reproducing female ;), it always ocurred to me that in human reproduction we really go more for “quality” in our offspring than quantity.
So many things fit into this: The fact that lactation usually prevents ovulation, menopause and the “good gay unce” hypothesis. With pregnancy and childbirth being so much more risky in humans than most animals, it makes sense to invest more in 5 or even 10 children than having 20 and losing most of them.
But I strongyl disagree with the author at one point:
Total nonsense. No way I could do without the help of the two most wonderful grandmothers in the world.
OK Greg, I will try a little harder, because I respect your insights into evolutionary theory but also think the grandmother hypothesis is idle speculation.
From the info in that post you referenced, and from what little I know about primate reproduction, there is not any evidence that female primates *ever* had the ability to ovulate past the age of 45. So you can’t argue that human females have evolved a shorter reproductive lifespan in response to selection pressure, and are reduced to speculating that “Extending fecundity is probably not that hard.” Since as far as we can tell that has not ever happened in humans, who knows? The more I learn about evolution, the more convinced I get that it is usually impossible to predict in advance whether a specific phenotype change is gonna result in increased fitness. The best you can do is rationalize it after the fact.
If you’re going to explain the grandmothers’ behavior as the adaptation, consider that this is what they would be doing anyway (if they had extra stuff after providing for their own children). Maybe it’s just the best thing they can do for their descendants after they run out of eggs.
Ya, I do tend to play Devil’s Advocate whenever people bring up adaptationist explanations for stuff (especially behavior) but that is because the instinct to look for purposes behind observations is so strong that I have to practice resisting it.