It's always been a bit of an evolutionary mystery why humans lives so long after peak fertility -- specifically, why women living so long after menopause.  The standard explanation is called the grandmother hypothesis, or the idea that a grandmother can devote resources to bringing up her grandchildren.  This always sounded a little too politically correct to be true, a little too in line with modern morals.

Here's a new paper that essentially argues that humanity, and especially post-menopausal women, owe their increased longevity to powerful, lecherous old men who mated with fertile, young women.  (And remember, the attraction goes both ways.)

Here's the abstract (my bolding):

Evolutionary theory predicts that senescence, a decline in survival rates with age, is the consequence of stronger selection on alleles that affect fertility or mortality earlier rather than later in life. Hamilton quantified this argument by showing that a rare mutation reducing survival is opposed by a selective force that declines with age over reproductive life. He used a female-only demographic model, predicting that female menopause at age ca. 50 yrs should be followed by a sharp increase in mortality, a “wall of death.” Human lives obviously do not display such a wall. Explanations of the evolution of lifespan beyond the age of female menopause have proven difficult to describe as explicit genetic models. Here we argue that the inclusion of males and mating patterns extends Hamilton's theory and predicts the pattern of human senescence. We analyze a general two-sex model to show that selection favors survival for as long as men reproduce. Male fertility can only result from matings with fertile females, and we present a range of data showing that males much older than 50 yrs have substantial realized fertility through matings with younger females, a pattern that was likely typical among early humans. Thus old-age male fertility provides a selective force against autosomal deleterious mutations at ages far past female menopause with no sharp upper age limit, eliminating the wall of death. Our findings illustrate the evolutionary importance of males and mating preferences, and show that one-sex demographic models are insufficient to describe the forces that shape human senescence.

Female hypergamy is such that once men could maintain social dominance over other men beyond the decline of their physical prowess -- through coalitions, culture, and technology -- it created a huge evolutionary/reproductive reward for men with extended reproductive lives.  Women inherited the resulting gains in longevity, though not the extension in fertility.

Why not?

Well, I assume it's because the passage of time plays to the strengths of high status men (not all men, mind you) -- since powerful men can accrue more power over time. But the passage of time cuts against the strengths of women -- older women consistently get undercut by younger ones.  Powerful men were swimming with the tide, while women were swimming against it.  (Low status men drowned.)

Anyhow, I have dubbed this "The Lecherous Grandfather Hypothesis".

"Out of the crooked timber of humanity no straight thing was ever made."

Here's the full paper.