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Thank you, Rod Stewart. Seriously.

Scientists say, in not exactly these words, that it is the old goats who keep the human race alive for longer.
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Earlier this year, Rod Stewart married model Penny Lancaster, promising to love and honor the latest version of his eerily uniform young-blonde-wife prototype. The new Mrs. Stewart is younger than Stewart's own young daughter, and at this rate, it's hard to believe the deeply lined rocker won't end up dating his own distant, blonde descendants. But it turns out, Rod, you are to be thanked. And the pre-menopausal ladies you callously jilted? They should thank you most of all. No, not because they don't have to sleep with you anymore, but for something much more important. Scientists say, in not exactly these words, that it is the old goats who keep the human race alive for longer.

Generally, it's thought that the more fertile you are, the likelier you are to survive. Additionally, the older you are, the higher your chances of dying. Common sense seems to bear this theory out. Yet there's an unexplained mystery about the length of women's lives. Once their ability to have children shuts down, there is no obvious reason for their biology to resist all the forces that conspire to take them down. Indeed, according to demographic models, women over 50 should hit a "wall of death." But they don't. Why not?

Scientists at Stanford and the University of California at Santa Barbara have developed a model to resolve the anomaly. The problem arises, they say, because accounts of the human lifespan have only focused on one or the other sex. If you look at both men and women together, the paradox disappears. The new model, published at the Public Library of Science, confirms the idea that, yes, for as long as people reproduce, they are better able to resist random harmful mutations that might kill them. Evolutionary forces have already weeded out the individuals who weren't so good at doing this, and accordingly they did not have many children to then pass their vulnerabilities on. Once people stop being fertile, all bets are off. There is little payoff for the species as a whole if everyone stays alive long after they can have children. This is why you see a decline in health after reproduction ends. The kicker is this -- women get a free ride into old age because they have the same genes as men who -- I'm looking at you, Rod -- have children later than women.

The fact that men's fertility tapers off around 60 or 70 explains why there is no sudden drop-off in the quality of life for men or women. Our lives taper off as well. How odd that you only need to have one sex to stay fertile in order for both to live longer lives.

In order to prove that much older fathers were not a rare occurrence or just a rock & roll legend, the researchers gathered data from many different cultures all over the world, including the Dobe !Kung in the Kalahari, the hunter-gatherer Ache in Paraguay and modern-day Canada. Examples of men who were over 55 and up to 70-years-old fathering children with younger women were found in all cases. In modern day Germany and Japan, the reproduction of men aged 65 was found to be the same as that of women aged 45. The finding was true for monogamous as well as polygamous societies. One of the ways that men end up being reproductive longer in a monogamous society like ours, say the researchers, is that they are more likely than women to remarry after divorce. Crucially, the researchers suggest that the pattern of very old dads was common in prehistoric times as well.

The new demographic solution is basically a mathematical theory, which means that even though it explains the facts nicely, it may not be what actually happened. Still if it is right, the fact that women survive at all beyond menopause is a byproduct of men's ability to father children into advanced old age, which, this is sticking in my craw, means that the human race really needs its randy geezers. As for the Penny Lancasters, they are taking it for the whole team.

What of the death-defying older women? Why don't they have children in their 70s? And why would they even evolve something as bothersome as menopause? One theory is that more of a woman's descendants will survive if she shuts down her own fertility and focuses on helping the children of her children. So even if older women got a leg up in the survival stakes because of older men, they play an indispensable role in perpetuating the human race when they become grandmothers. Thanks, grandma.