An interesting PLOS ONE paper from a few years back has been brought to my attention Why Men Matter: Mating Patterns Drive Evolution of Human Lifespan, by Shripad D. Tuljapurkar, Cedric O. Puleston, and Michael D. Gurven.
They begin with a brief explanation of the evolutionary theory of senescence, a subject with which I am intimately familiar. Hamilton (considering only female demography) showed that selection against a rare autosomal mutation that reduces survival at some age is proportional to that female’s survival-weighted reproductive output past that age. A mutation that caused trouble after the end of reproduction will not be purged by selection – at all. By this argument, since menopause happens, everyone should be dead by 55. Hey, it works for salmon and marsupial mice.
But we aren’t. There have been several attempts to explain this: Kristen Hawkes’ grandmother hypothesis, and various arguments involving codgers transferring resources or information. It’s hard to make the numbers work out. Tuljapurkar et al argue that dirty old men are the answer: as long as they keep reproducing at a significant level when past the age of female menopause, there is some selection against those late-onset deleterious mutations.
I doubt if they’re right, because I think the societies with lots of old goats reproducing are somewhat atypical, not too likely to have been the pattern before agriculture. Although the Australian Aborigines manage it. I could be wrong. There was an illuminating graph in this article, better than a Bo Tree:
The black line is male fertility as a function of age in France in 1980: blue dots are Pakistan 1984: red dashes are Cameroon 1964. Tuljapurkar et al mention that Cameroon’s distribution is typical of high-fertility polygynous societies. Like I said before.
Even if dirty old men are not the main or only factor selecting for longer human lifespans, they must push in that direction. This may well explain the black-white mortality crossover. Unfortunately, this kind of reproductive pattern decreases the strength of selection in early life, which ought to increase child mortality, all else equal. It also increases the mutation rate by up to 50%, which must really interfere with the most complex adaptations. Tuljapurkar and company weren’t thinking about the long-term effects of the increased mutation rate, but as far as we can tell, nobody ever has.