The egg that eventually became you once lay in your grandmother. This giddy fact(oid) happens because, in the womb, women develop all the eggs they will ever use. The supply gradually declines through the years to the point where no more eggs remain, no more children can be sired: menopause.
But why should fertility grind to a halt several decades before the body gives out? Why lose the ability to reproduce? If life is all about making babies and turning them into baby-making machines themselves (speaking biologically, not philosophically), why run out of eggs but carry on living?
Menopause has puzzled many scientists for many years. Some explanations say menopause evolved so that either mothers or grandmothers live long enough to raise their (grand)children, while others suggest that menopause prevents competition between mothers and daughters-in-law. Some think that we just run out of eggs.
A team of anthropologists, primatologists and biologists recently demonstrated that while human lifespan increased at some point in our evolutionary past, our reproductive lifespan has not caught up. This lower death rate ‘reveals’ the menopause.
“Unlike other primates women tend to have a long post-reproductive life. Even before modern medicine, many women lived for 30 to 35 years after their last child was born,” said the study’s lead author Professor Susan Alberts of Duke University and the National Evolutionary Synthesis Center in a press release.
However, in an interview with The Tangled Woof of Fact, Alberts cautioned: “I don’t think [our paper] settles the debate about how menopause evolved at all. I think what our data do is put a nail in the coffin for the idea that humans aren’t different to other primates.”
The researchers built on a previous paper published in earlier in 2013 which looked at the end of reproductive life for a bunch of animals in captivity (and was nicely covered by Carl Zimmer). This new study looks at fertility and death among wild primates. Alberts described their aim as: “filling out that comparative landscape.”
To understand the pace of fertility decline, the researchers calculated the probability that each female primate’s birth would be her last, for any given age, and compared this to the risk of dying.
“We were able to compare the rate of ageing in the reproductive system with the rate of ageing in the rest of the body,” Alberts said.
The study looked at seven wild non-human primate species and the Dobe !Kung hunter-gatherers, who live in southern Africa’s Kalahari desert. The !Kung data come from a time when they had little access to modern medicine or contact with other groups, so can be thought to represent the ‘natural’ human pace of reproducing and dying.
Most non-human primates reproduce until they die; only a small minority hit what we might term a menopause. Many women, on the other hand, live well past menopause. This seems to show that the human reproductive system ages at a faster pace than the rest of the body. In the parlance, this is called reproductive senescence.
So, what is special about humans? Why do our bodies age so slowly compared to other primates? Why haven’t reproductive systems ‘caught up’?
Alberts, like a number of researchers, thinks the reason for our menopause is linked to our need to be fed by other people for a long time, even after weaning.
“Anywhere nutritional dependence extends well beyond weaning, I think, would be a place to look for the evolution of accelerated reproductive senescence,” Alberts said.
“In humans, of course, we get weaned between two and five years of age depending on what society we live in, and then we’re nutritionally dependent for another two decades. I mean, that’s insane.”
This hints, nods and winks towards the most famous of evolutionary explanations for menopause: the grandmother hypothesis. This is the idea that women who are no longer physically able to reproduce instead spend their post-menopausal years helping to bring up their grandkids.
Although this is only a hint, a nod and a wink to grandmothers, the ‘true’ explanation may require a wider range of people – the proverbial village that raises a child.
“What that means is that you have a situation where you can’t wait until the current [child] is nutritionally independent before you have the next one,” said Alberts. “So what humans do is we pile them up and we end up with whole sets of dependent offspring that we can’t raise on our own.”
“A single person can’t do it: it requires a community. That’s the key, I think to understanding the evolution of reproductive senescence in humans.”
Reference: Alberts, Altmann, Brockman, Cords, Fedigan, Pusey, Stoinski, Strier, Morris and Bronikowski – 2013. Reproductive aging patterns in primates reveal that humans are distinct. PNAS http://dx.doi.org/10.1073/pnas.1311857110