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10 The Evolution of the Human Life Course
Pages 175-211

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From page 175...
... Alternatively, it may be the result of other selective forces, such as the costs of maintaining viable oocytes for many decades. Even if menopause is the result of other selective forces, the theory may still account for the extension of life span beyond the reproductive period.
From page 176...
... HUMAN AND NONHUMAN PRIMATE LIFE HISTORIES: FUNDAMENTAL CHARACTERISTICS Mortality and Longevity Survival curves for four traditional groups and chimpanzees are presented in Figure 10-1. The Ache are a hunting and gathering group, living in the subtropical forests of eastern Paraguay, who made first peaceful contact with outsiders in the 1970s and now practice a mixed economy of hunting, gathering, horticulture, and wage labor (see Hill and Hurtado, 1996, for a detailed description of their way of life and demography as hunter-gatherers; for further information on diet and activities, see Hawkes et al., 1982; Hawkes et al., 1987; Hill and Kaplan, 1988a, b; Hill and Hawkes, 1983; Hill et al., 1985; Kaplan and Hill, 1985; Hurtado et al., 1985~.
From page 177...
... Although sample size and methods of data collection vary among the four human groups, the survival curves show remarkable convergence. Although infant mortality rates vary, with Hiwi being the highest and Yanomamo the lowest, adult mortality rates between the ages of 20 and 45 are almost identical, about 1.5 percent per year.
From page 178...
... Even gorillas, with much larger body sizes, do not live much longer than chimpanzees and have an adult mortality rate of about 5 percent per year (Harcourt and Fossey, 1981~. The most reliable estimates of adult mortality rates available for a pre-contact hunting and gathering group are derived from Ache research (Hill and Hurtado, 1996)
From page 179...
... Detailed information on the foraging behavior of children is also available for the Hadza, hunter-gatherers living in a mixed savanna woodland habitat in the Eastern Rift Valley of Tanzania (see Woodburn, 1968, 1972, 1979, for general ethnographic information; for data on food production by age, see Blurton Jones, 1993; Blurton Jones et al., in press; Blurton Jones et al., 1994a, b; Hawkes et al., 1989, 1991, 1995, 1996~. In the above series ofpapersbyBlurtonJones,Hawkes, and O'Connell, the authors report that Hadza children can be very productive, especially when compared to Mung children.
From page 180...
... Quantitative data on food production and food consumption through the life course (measured in units of calories per day) are available for three different traditional groups: Piro, Machiguenga and Ache (see Figures 10-3a - 10-3c; and Kaplan, 1994, for details)
From page 181...
... Net productivity _ ,~ / Net deficit _~ 2000 1 000 O ~ -1 000 _ _ - it, / Net surplus 0 5 10 15 20 25 30 35 40 45 Age FIGURE 10-3c Ache food production and consumption by age: both sexes combined. SOURCE: Kaplan (1994)
From page 182...
... When people eat fruits, these fruits tend to be large and ripe, whereas nonhuman primates feed on a much larger array of small and unripe fruits as well. The bulk of the food acquired by human foragers is derived from difficult-toextract, nutrient-dense plant foods and hunted game.
From page 183...
... Bock, who worked with villagers in the Okavango delta who practiced a mixed economy of hunting, fishing, gathering, horticulture, and animal husbandry, found that mongongo cracking rates peak at age 35 for women (Bock, 1995~. The most important plant food among the Ache is palm starch.
From page 184...
... in any nonhuman primate, hunted and fished foods account for between 15 and 100 percent of total calories consumed among human foragers (Kelly, 1995:table 103.1~. Although there is no comparative, quantitative database on the factors affecting hunting ability in humans, my own observations hunting with four South American groups suggest that hunting, as practiced by those peoples, is a very skill-intensive activity.
From page 185...
... , the authors show that foraging return rates and especially access to fruits close to camp sites are the critical determinant of the higher food acquisition by Hadza children. Not only are there more fruits close to Hadza camps than close to Mung camps but also the environment near Mung camps is more dangerous for children due to poor long-range visibility (ibid.~.
From page 186...
... As a result, human children are provisioned. The Relationships Between Feeding Ecology, Reproductive Ecology and Life Span Table 10-2 summarizes some major differences in the life-history characteristics of humans and chimpanzees interrelated by the theory.
From page 187...
... At some stage in the life course it is likely that a family will contain children of about 15, 11, 7, and 3 years of age. A second important difference between humans and other primates, including the great apes, is the energetic support for lactation.
From page 188...
... SOURCE: Lancaster (1986~. Reprinted by permission The storage of fat during pregnancy as a preparation for lactation may be a direct result of our feeding ecology.
From page 189...
... The exceptionally low adult mortality rates before age 60 appear an integral component of the skills-based food niche. A GENERAL MODEL FOR THE EVOLUTION OF LIFE HISTORIES The above discussion presented a specific theory for the evolution of human life-history characteristics.
From page 190...
... At each age, an individual's income will be a function of his or her embodied capital. Income can be invested directly in reproductive effort or in
From page 191...
... Thus, the optimization problem acted upon by natural selection is to allocate lifetime income among investments in future income, survival, and reproductive effort at each age so as to maximize total allocations to reproduction. The optimum strategy will be the one that maximizes resources for reproduction over the life course (see Charnov, 1993; Kirkwood, 1981; Kozlowski and Weigert, 1986; and Stearns, 1992, for theoretical treatments)
From page 192...
... Some organisms, such as bivalve mollusks, tortoises, and porcupines, apparently benefit significantly from allocations to predator defense and live long lives. Feeding niche appears to interact with the benefits to investments in mortality reduction.
From page 193...
... These general results can be applied to the specific model of human lifehistory presented above. The nature of our feeding niche increases the value of both one' s own and parental investment in income-related capital.
From page 194...
... . The interaction of increased returns to income-related capital and greater resistance to predation and disease may be directly responsible for our long life span and delayed senescence.7 In the light of this model, we should not expect to see people living longer than they are productive under traditional conditions.
From page 195...
... Those who assume the existence of a postreproductive period have addressed the problem from the perspective of the evolution of menopause. Taking the length of the human life span as a given, they have asked what kinds of selective forces could result in the evolution of menopause.
From page 196...
... One possibility is that the ancestral condition is a somewhat longer reproductive period and a shorter life span. Selection, due to an increased value of grandparenting, could direct additional resources toward longevity and investment in descendant kin, at the expense of shortening the reproductive period.
From page 197...
... This system is consistent with the feeding ecology model discussed above. Thus, it is possible that a long life span evolved, even though an increase in the reproductive period did not, due to different costs and benefits.
From page 198...
... An understanding of men's behavior may help solve the riddle of long life and menopause. HISTORICAL, CURRENT, AND FUTURE TRENDS IN MORBIDITY AND MORTALITY The general life-history model presented above predicts that increased returns to investments in embodied capital affecting either income or survival will
From page 199...
... However, before the discussion of those implications, it is important to consider the evolution of short- and long-term responses to ecological variation. The traditional view of evolutionary processes underlying adaptation is that in a given environmental context, natural selection favors certain genetic variants over others, with each genetic variant corresponding to a phenotypic trait.
From page 200...
... The central prediction of the life-history model presented above is that payoffs to investment in income-related capital interact positively with payoffs to investment in survival in determining allocations to reproductive effort and embodied capital. One theory of the demographic transition (the dramatic reduction in fertility that occurred in Europe and America about 100 years ago)
From page 201...
... In modern labor markets, increased education is not only associated with increased income but also with higher rates of income growth through the life course (Mincer, 1974; see Figure 10-11~. According to the life-history model, the increased value of investments in education and growth in income through the life course should favor increased investment in longevity.
From page 202...
... It may be that large, future increases in the healthy human life span will require major manipulation of our evolved allocation system, either through genetic engineering or chemical interventions. SUMMARY AND CONCLUSIONS This paper has addressed the evolution of the human life course from the perspective of competing allocations to reproduction, growth, skill development, health, and maintenance.
From page 203...
... Two distinct possibilities regarding the evolution of the postreproductive period were considered. One is that menopause evolved to facilitate postreproductive investment in offspring.
From page 204...
... 1975 Human Capital, 2nd ed. New York: Columbia University Press.
From page 205...
... 1996 Biological bases for plasticity during aging of individual life histories.
From page 206...
... Finch, C.E., and M.C. Pike 1996 Maximum life span predictions from the Gompertz mortality model.
From page 207...
... Human Ecology 15:163-187. 1990 Seasonality in a foraging society: Variation in diet, work effort, fertility and the sexual division of labor among the Hiwi of Venezuela.
From page 208...
... A test of an optimality model and a new theory of parental investment in the embodied capital of offspring. Human Nature 6:325-360.
From page 209...
... 1994 Evolution of time preference by natural selection. American Economic Review 84(3)
From page 210...
... Stearns, S 1992 The Evolution of Life Histories.
From page 211...
... C 1957 Pleiotropy, natural selection and the evolution of senescence.


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