The Answer To The Ultimate Question Of Sex, The Death, And Men Is 42
The paper proposes a solution to a conundrum in the study of human lifespan: why don’t we drop dead soon after the age of last female reproduction?
Our understanding of the evolution of lifespan suggests that we have no defense against mutations that occur after we reach the end of our reproductive lives. As a result we expect a rapid increase in mortality — a “wall of death” — just after female menopause.
The authors show that the standard practice of tracking only female life histories leads to mistaken conclusions about the forces that shape human evolution. The reason is that men’s and women’s age patterns of fertility differ in important ways.
The paper brings together data from hunter-gatherer populations to show that male reproduction begins and ends later than women’s, and declines much more gradually. In many populations, historically and even today, some fraction of men continue to father children into their 60s and 70s with younger women.
In some groups, most notably Australian aboriginal and African polygynous societies, late-age male reproduction is common. In many hunter-gatherer societies, which may tell us most about how our ancestors once lived, men begin to reproduce a few years later than women of the same age and they typically continue to father children for several years after the age of female menopause due to the marriage gap in the ages of couples.
The marriage gap, in which older men marry younger women, appears to be a near-universal human trait.
PLoS One 2(8): e785.doi:10.1371/journal – http://www.plos.org/press/pone-02-08-tuljapurkar.pdf
Why Men Matter: Mating Patterns Drive Evolution of Human Lifespan.
Tuljapurkar SD, Puleston CO, Gurven MD (2007)
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.