. "11 How Grandmother Effects Plus Individual Variation in Frailty Shape Fertility and Mortality: Guidance from Human-Chimpanzee Comparisons--Kristen Hawkes ." In the Light of Evolution IV: The Human Condition. Washington, DC: The National Academies Press, 2010.
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In the Light of Evolution Volume IV: The Human Condition
Long postmenopausal survival is a characteristic of our species. The use of life expectancy to compare human populations can obscure this fact because high infant and juvenile mortality kept all national life expectancies below 50 until the 20th century (Oeppen and Vaupel, 2002). As historical demography shows, girls that survived childhood usually lived long past menopause in previous centuries (Keyfitz and Fleiger, 1968). Hunter-gatherer survival curves are especially instructive (Howell, 1979; Hill and Hurtado, 1996; Early and Headland, 1998; Blurton Jones et al., 2002; Hill et al., 2007). They document characteristic human longevity in the absence of agriculture, public health institutions, and scientific medicine, all of which emerged long after the initial evolution of our species (Hawkes and Blurton Jones, 2005; Gurven and Kaplan, 2007). Distinctive and at first puzzling human postmenopausal survival was addressed in classic papers that used evolutionary theory to explain why living things grow old.
G. C. Williams (1957) laid out demographic reasons why declines in adaptive performance with increasing adult age emerge from the forces of natural selection. Because life is risky, cohorts inevitably diminish across adulthood. Consequently, the forces of selection weaken with age as fewer remain to be affected by it at older ages. Williams explained how the same forces result in different rates of senescence among species that reproduce more than once depending on two aspects of life history. First, when background mortality risk is lower, more individuals survive to older ages and selection against senescence is stronger. Second, selection against senescence is also stronger when the potential fitness-related payoffs to survivors increase with age. He illustrated the latter effect with the slow senescence of indeterminate growers that continue to increase in size and rate of egg production throughout adulthood.
Concluding that evolutionary life history theory predicts no post-reproductive period in normal life spans, Williams then addressed the apparent contradiction posed by survival past menopause in our own species by observing that older women still investing in descendants are not literally postreproductive. Hamilton (1966) mathematically modeled the tradeoffs nominated by Williams and demonstrated that the forces of selection shape mortality schedules to converge asymptotically with the age when reproduction ends. This process leaves, as Williams had surmised, few if any postreproductives. Because “much the best” (Hamilton, 1966, p. 27) demographic data are available on humans, Hamilton used a human population to explore the fit of observation with theory. This required him to explicitly confront the apparent discrepancy in the case of humans (Hamilton, 1966, p. 37):