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10 The Evolution of the Human Life Course Hillard Kaplan Introduction This paper presents a theory of evolution of the human life course. Compared to other primates and mammals, there are at least three distinctive characteristics of human life histories: (1) an exceptionally long life span, (2) an extended period of juvenile dependence, and (3) support of reproduction by older postreproductive individuals. Because most hominid evolution occurred in the context of a hunting and gathering life style and because all well-studied hunting and gathering groups exhibit those three characteristics, the theory considers the aspects of the traditional human way of life that might account for their evolution. It proposes that those three features of the human life course are interrelated outcomes of a feeding strategy emphasizing nutrient-dense, difficult-to-acquire foods. The logic underlying this proposal is that effective adult foraging requires an extended developmental period during which production at young ages is sacrificed for increased productivity later in life. The returns to investment in development depend positively on adult survival rates, favoring increased investment in mortality reduction. An extended postreproductive, yet productive, period supports both earlier onset of reproduction by next-generation individuals and the ability to provision multiple dependent young at different stages of development. A postreproductive period depends upon menopause. Menopause may have evolved to facilitate postreproductive investment in offspring. 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.
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The paper begins with a basic description of available data on longevity in traditional hunting and gathering societies and the age profile of food production and consumption. These data are compared to information available on nonhuman primates with particular emphasis on chimpanzees, our closest living relatives.1 This discussion is followed by consideration of the comparative feeding and reproductive ecologies of humans and nonhuman primates. A model is then presented to outline the major tradeoffs involved in life-history evolution. The model shows that investments in foraging efficiency and mortality reduction coevolve and affect the age pattern of investments in reproduction. Several different approaches to the evolution of menopause are then considered. The paper concludes with a discussion of the implications of the theory for historical, current, and future trends in human development and longevity. 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 Aché 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). The Hiwi live in the Venezuelan savanna and rely primarily on hunting and gathering roots for their subsistence (for ethnographic information on the Hiwi, see Hurtado and Hill, 1987, 1990, 1992; Hurtado et al., 1992). The !Kung were hunter-gatherers with various degrees of contact and economic relationships with other groups until the 1970s and now practice a mixed economy of hunting, gathering, farming, and wage labor (for ethnographic information on the !Kung, see Blurton Jones, 1986, 1987; Blurton Jones and Konner, 1976; Blurton Jones et al., 1994a, b; Blurton Jones et al., 1989; Draper, 1975, 1976; Draper and Cashdan, 1988; Harpending and Wandsnider, 1982; Howell, 1979; Konner and Shostak, 1987; Konner and Worthman, 1980; Lee, 1979, 1984, 1985; Lee and DeVore, 1976; Schrire, 1980; Wiessner, 1982a, b; Wilmsen, 1978, 1989; Yellen, 1976). The Yanomamo practice a mixed economy of hunting, gathering and horticulture; many Yanomamo groups have yet to make peaceful contact with outsiders (Chagnon, 1974, 1983, 1988; Hames, 1983, 1992; 1 Although gorillas and bonobos (pygmy chimpanzees) may be as closely related to humans as common chimpanzees, the demographic and behavioral data on the latter are much more complete.
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Figure 10-1 Age-specific probabilities of survival among human foragers and chimpanzees. SOURCE: !Kung (Howell, 1979); Yanomamo (Melancon. 1982); Aché (Hill and Hurtado, 1996); Hiwi (Hill and Hurtado, unpublished data from Kim Hill); chimpanzees (Goodall, 1986, and Courtenay and Santow, 1989). Melancon, 1982). The chimpanzees are those living in the Gombe nature reserve (for dietary and life-historical information on chimpanzees at Gombe, see Courtenay and Santow, 1989; Goodall, 1986; Silk, 1978, 1979; Teleki, 1973; Wrangham, 1974, 1977; Wrangham and Smuts, 1980). 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. For that reason the survival curves are parallel to one another during the adult period. Chimpanzee survival curves, however, diverge dramatically from the human curves, due to a quite distinct adult mortality profile. For example, while both Hiwi and chimpanzees have about equal probability of reaching age 15, the conditional probability of reaching age 45, having reached age 15, is near zero for chimpanzees in the wild and about 75 percent among the Hiwi (see also Lancaster and King, 1992, for supporting data from other groups). Adult mortality rates among chimpanzees over age 25, living at the Gombe reserve, are about 7.9
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percent per year (Goodall, 1986; Courtenay and Santow, 1989), about five times as high as among the four traditional human societies. 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 Aché research (Hill and Hurtado, 1996), because of the research focus on producing accurate measures of age and accounting for all adults that lived during the twentieth century. Figure 10-2 shows the age-specific mortality rate of Aché males and females. Adult mortality rates remain low and do not rise significantly until the seventh decade of life, where the rate climbs to 5 percent per year and reaches 15 percent per year by age 75. It should be mentioned that the data displayed in Figure 10-2 deviate somewhat from the age-specific mortality profile predicted by the Gompertz model (see Finch et al., 1990; Finch and Pike, 1996). According to that model, which is quite robust in predicting the mortality profiles of many animal populations (see references cited in Finch et al., 1990), human adult mortality rates are expected to double about every 8 years (ibid: 903). The slow rate of increase in mortality during early and middle adulthood estimated for the Aché may be due to small sample size. Alternatively, it may be that age-related increases in mor Figure 10-2 Aché age-specific probability of death, smoothed with logistic regression. SOURCE: Hill and Hurtado (1996: Fig. 6.2). Copyright 1996 by Water de Gruyter, Inc., New York.
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tality due to physical deterioration are swamped by causes of death due to accidents, snake bite, warfare, and predation by jaguars that impact on all adult age classes equally and may even occur more frequently in young adults (see Hill and Hurtado, 1996: table 5.1). In any case, once a child reaches adult age, the prospect of surviving to a reasonably old age is high. For example, a woman who reaches the average age of first reproduction (age 19) has about a 50 percent chance of reaching age 65.2 This suggests that living well past the age of last reproduction is a common experience for human females. This survival probability distinguishes our species from almost all other mammals, with the notable exception of some whales (discussed by Austad, in this volume) and contrasts markedly with chimpanzees. Feeding Ecology and the Life Cycle of Productivity Although there is a great deal of variability in the diets of both hunting and gathering groups and nonhuman primate species, there appears to be a fundamental difference in the age schedules of production and consumption between humans and their primate relatives. Human children remain dependent on their parents until well into their teen years and sometimes until they are over 20 years old. Data collected with !Kung San also indicate that children under age 15 acquire very little food (Draper, 1976:209-213; Draper and Cashdan, 1988; Lee, 1979:236), spending less than 3 minutes per hour engaged in productive labor. Hill and Hurtado's data on Hiwi children (Kaplan, Hill, Hurtado, and Lancaster. unpublished work) show that boys do not produce as much food as they consume until about age 18 and girls do not do so until they are postreproductive. 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 of papers by Blurton Jones. Hawkes, and O'Connell, the authors report that Hadza children can be very productive, especially when compared to !Kung children. Nevertheless, Hadza girls do not produce as much as they consume until about 15 years of age. and boys produce about half as much as they consume through 18 years of age (Blurton Jones et al., in press: figure 5). In contrast with the low productivity of children in hunter-gathering groups, postreproductive and middle-aged people, especially women, appear to work very hard and produce much food. Among the Hadza, postreproductive women 2 While estimates derived from some prehistoric mortuary samples show much lower adult survival rates, there is good reason to believe that inaccuracies due to aging of materials and the sampling properties of the distribution of found remains make them unreliable.
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spend 22-52 percent more time in food acquisition than reproductive-age women (depending on the season), and 90-275 percent more time than unmarried girls (Hawkes et al., 1989: figure 2 and table 1). Among the !Kung, while work effort appears to decline with age during the adult years, people over 60 work almost as many hours as younger adults (Lee, 1979: table 9.5). 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 Aché (see Figures 10-3a - 10-3c: and Kaplan. 1994, for details). The Piro and Machiguenga practice a mixed economy of swidden horticulture, hunting, fishing, and gathering. There is considerable similarity in the age profiles of the three groups. First, children produce much less than they consume, and production does not exceed consumption until 18-20 years of age. Childhood and even adolescence are characterized by very low rates of food production. Second, production exceeds consumption well past the reproductive period into old age. This is particularly evident among the Piro and Machiguenga. Unfortunately, sample sizes for older Aché men and women are extremely low, due to high rates of death associated with disease at first contact. However, data on Aché men show that they produce about twice as much as they consume in their fifties, but in their sixties they produce about a third of what they consume. This pattern contrasts markedly with age profiles of production among non Figure 10-3a Machiguenga food production and consumption by age: both sexes combined.
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Figure 10-3b Piro food production and consumption by age: both sexes combined. Figure 10-3c Aché food production and consumption by age: both sexes combined. SOURCE: Kaplan (1994).
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human primates. Virtually all nonhuman primates follow the standard mammalian pattern. The period of infancy is one of complete nutritional dependence on the mother. The second, juvenile period, from weaning to the onset of reproduction, is characterized by almost exclusive self-feeding. There is no significant period of nonlactational parental provisioning among nonhuman primates. The third, adult period begins with reproduction but includes no significant period of postreproductive productivity before ending in death. These differences between humans and nonhuman primates are summarized in Table 10-1. My proposal is that these differences are linked to dietary differences. A close examination of the feeding ecology of human hunter-gatherers. when compared to that of nonhuman primates, yields some revealing patterns. The major difference between human and nonhuman primate diets is in the importance of nutrient-dense, difficult-to-acquire (i.e., skill- and/or strength-intensive) food resources (see Figure 10-4). While the diets of nonhuman primates vary considerably by species and by local ecology, most feed, to various degrees, on leaves, fruits, and insects, supplemented in some cases by small amounts of hunted meat and tree gums (Oates, 1987; Terborgh. 1983). Humans, in contrast, rarely feed on leaves. When people do consume leaves, it is as a low-calorie supplement to calorie-dense foods (David Tracer, personal communication) as a source of micronutrients. Humans also avoid most fruits consumed by primates living in the same area. 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-to-extract, nutrient-dense plant foods and hunted game. Calorically, the most important plant foods for humans are roots, seeds, palm fiber, and nuts. Among the Hadza, roots are the most important plant food. The \\ekwa roots (Vigna frutescens), which provide the bulk of the carbohydrate calories in the diet, are found deep in rocky soil, from which ''extraction is a lengthy subterranean jigsaw puzzle, sometimes involving the removal of heavy boulders and encounters with scorpions" (Blurton Jones, 1993). Although Hadza researchers do not report actual amounts of roots acquired as a function of age, they do report return rates per hour of work for \\ekwa root digging. Until about age 10, children acquire less than 200 kcal/hr, and then returns increase steadily by about 125 kcal/hr/yr until the age of 18 when they acquire about 1 100 kcal/hr TABLE 10-1 Life History Stages Mammals/Primates Traditional Humans Infancy Infancy Independent prereproductive. juvenile Dependent, prereproductive juvenile Adult, reproductive Adult, reproductive Postreproductive, productive Frail elderly (very short until recently) Death Death
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Figure 10-4 The feeding ecology of humans and other primates. of work (Blurton Jones et al., in press: figure 2). Reproductive-aged and postreproductive women acquire 1500 and 1670 Kcal/hr, respectively. A similar pattern is found among Hiwi hunter-gatherers, for whom roots are also the most important plant food. Among some !Kung groups, mongongo nuts are reported to be the plant food staple (Lee, 1979). While the nuts are easy to collect, several factors appear to limit the productivity of children (see Blurton Jones et al., 1989, 1994a,b for an in-depth analysis). First, mongongo nut groves are often found quite distant from water sources (about 10 km) where camps are located (Blurton Jones et al., 1994a,b). This requires a great deal of endurance and the ability to walk far without much water (ibid.). In addition, extraction of nut meat requires skill. According to experimental data on nut-cracking rates (Blurton Jones et al., 1994a), most children under the age of 9 are unable to crack the nuts safely. Children aged 9-13 cracked 120 nuts per hour, teens aged 14-17 cracked 241 nuts per hour. and adults cracked 314 nuts per hour. 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 Aché is palm starch. Extraction of palm starch requires felling the tree, cutting a vertical window down the length of the trunk to expose the pulp, and then pounding the pulp into mush. This is a difficult task involving both strength and skill, and women do not reach peak productivity at palm-fiber extraction until age 35 (A.M. Hurtado. unpublished data). Again, Aché girls less than 15 years of age rarely pound palm fiber. Seeds, an important plant food staple in Australia and the North American
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Great Basin (e.g., O'Connell and Hawkes, 1981; O'Connell et al., 1983; Steward, 1938), also require much processing to extract the nutrients (Simms, 1984). Meat is also an important part of human diets. Whereas meat accounts for no more than 5 percent of total caloric consumption (and usually much less) 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. Because people are slow runners, they rely on knowledge of prey behavior to find and kill prey. Conversations with men among the Aché, Piro, Machiguenga, and Yora foragers suggest to me that they have detailed knowledge of the reproductive, parenting, grouping, predator avoidance, and communication patterns of each prey species, and this takes decades to learn. For example, in a test with wildlife biologists, an Aché man could identify the vocalizations of every bird species known to inhabit his region and claimed to know many more, which the biologists have yet to identify (Kim Hill, personal communication). After most hunts, details of the hunt and the prey's behavior are discussed and often recounted again in camp. Even the stomach and intestinal contents of the animal are examined to determine its recent diet for future reference. In addition, knowledge of predator behavior may also be very important. Villagers in Botswana reported to me that one reason why teens hunt little is because they are at risk of predation themselves. According to some informants, the ability to detect potential predators such as lions, hyenas, and leopards and then escape them requires years to learn. It should be mentioned, however, that available empirical data do not allow us to assess the relative impacts of skill, knowledge, strength, endurance, and ambition on hunting returns. Those impacts may vary across ecologies and individuals. Nevertheless, the age patterning of hunting success is striking. Figure 10-5 shows the age distribution of hunted calories acquired per day among the Aché. Fifteen- to seventeen-year-old boys acquired 440 calories of meat per day, 18- to 20-year-olds acquired 1,530 calories, and 21- to 24-year-olds acquired 3,450 calories, whereas 25- to 50 year olds acquired about 7,000 calories of meat per day. The fourfold increase between 18 and 25 years of age exists in spite of the fact that by age 18, young men are hunting about as much as fully adult men. This pattern is not unique to the Aché. From independent samples acquired in different !Kung camps, both Lee (1979) and Draper (1976) report that men under age 25 acquired very little meat and were considered incompetent hunters. Among the Hadza, although boys spend much time pursuing game, their returns are quite low. Blurton Jones et al. (1989) report that during 31 observation days the total meat production for Hadza boys was about 2 kg, mostly composed of small-to-medium-sized birds. This is less than the daily production of a single adult Hadza man, who acquires a mean of 4.6 kg per day (Hawkes et al., 1991).
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Figure 10-5 Aché male hunting acquisition. Fruit collection, in contrast, is the least skill-intensive activity in human foraging. Fruits are also the most important food acquired by children. In fact, most variability in children's food acquisition, both within and among hunting and gathering societies, appears to be due to access to fruits. In an insightful series of papers comparing !Kung and Hadza foraging (Blurton Jones, 1993; Blurton Jones et al., 1989, 1994a, b, and in press), 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 !Kung camps but also the environment near !Kung camps is more dangerous for children due to poor long-range visibility (ibid.). In addition, Hadza children acquire more food and spend more time foraging during seasons when fruits are abundant (Blurton Jones, 1993; Blurton Jones et al., in press; Hawkes et al., 1996). In fact, Hadza children can provide as much as 50 percent of their total calories when fruits are in season (Blurton Jones et al., 1989). Similarly, one area (Bate) where !Kung children were reported to forage more often did have fruit and nut trees nearby (Blurton Jones et al., 1994b:205). Fruits also explain dramatic variation in Aché children's foraging. When fruits are in season, food production increases more than fivefold for children under age 14 (Figure 10-6). For older teens who are stronger and more skilled, the effect is less dramatic. The effects of ease of acquisition also apply to meat. When meat resources are collectable, children can also be very productive. For example, among the Machiguenga and Piro, streams are frequently dammed and poisoned with roots.
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the result of the emergence of skills-based labor markets as the dominant economic institution (Kaplan, 1996; Kaplan et al., 1995). This theory proposes that payoffs to investment in education increased radically with the emergence of labor markets and technological growth spurred by the industrial revolution. As a result, parents lowered fertility to invest in more skilled children. The theory can be extended to consider the relationship between investments in income-related educational capital and investments in mortality reduction. 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. The increased investments in public and private health that we have witnessed in the past century may be explainable as direct outgrowths of increased payoffs to investment in skill. As a corollary, the improvements in health and survival also increased the value of investments in income growth, due to the increased duration of returns from those investments. Figure 10-11 Median annual earnings of full-time, full-year male workers in 1985 as a function of education and age. SOURCE: U.S. Bureau of the Census (1985).
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This view differs considerably from standard demographic transition theory, which explains fertility reduction as a result of reductions in infant and juvenile mortality. The standard theory sees fertility reduction as an equilibrating response to maintain population stability in the face of changing mortality regimes. The capital-investment theory explains lower fertility, increased survival rates, and increased investment in skills as coordinated responses to a changing economic system. The same logic predicts the observed correlation between old-age survival and education found today. Health-promoting and health-reducing behaviors (exercise, diet, cigarette smoking, drug and alcohol use) are closely associated with education. Perhaps the reason for this association is not the increased information available to educated people but the increased value of longevity associated with an increase in income growth through the life course (of course, many other possible causal processes may be responsible for this association). Because morbidity associated with behavior represents a significant portion of resources spent in health care, an understanding of the factors determining the relative values of present consumption and future longevity is of great practical importance. Finally, it is also the case that access to food resources is virtually unlimited for many people today. This food availability is outside the range of anything experienced by traditional peoples in the past. This availability may mean that our evolved allocation mechanisms are not designed to respond to unlimited food access. Perhaps the increases in longevity and the increased health of very old people that have occurred in the last several decades are at the bounds of our adaptive flexibility. Even though we have enough food energy to allocate additional resources to maintenance, we appear to store that energy as fat rather than to use it to prevent aging from occurring. Our evolutionary history probably did not design our allocation system to respond adaptively to the virtually unlimited food supply and ability to combat illness through medicine, characteristic of modern developed nations. 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. Compared to other primates and mammals, there arc three distinctive characteristics of human life histories: (1) an exceptionally long life span, (2) an extended period of juvenile dependence, and (3) support of reproduction by older postreproductive individuals. The theory presented here proposes that those three features of the human life course are interrelated outcomes of a feeding strategy emphasizing nutrient-dense, difficult-to-acquire
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foods. The logic underlying this proposal is that for humans, effective adult foraging requires an extended developmental period during which production at young ages is sacrificed for increased productivity later in life. The returns to investment in development depend positively on adult survival rates, favoring increased investment in mortality reduction. An extended postreproductive, yet productive, period supports both earlier onset of reproduction by next-generation individuals and the ability to provision multiple dependent young at different stages of development. Two distinct possibilities regarding the evolution of the postreproductive period were considered. One is that menopause evolved to facilitate postreproductive investment in offspring. The other is that reproductive senescence evolved due to the costs of maintaining viable oocytes and that increased longevity evolved, in spite of menopause, to support the reproduction of descendants. This theory was developed as part of a more general theory of the evolution of life histories. Two major tradeoffs were considered. First, resources can be invested in either current or future reproductive effort. Investments in future reproductive effort include both those that enhance survival and increase future income (in a general sense). Age-specific allocations that maximize the lifetime allocation to reproductive effort will be favored by natural selection. Second, there is a tradeoff between quantity and quality of offspring. The specific model of human life-history evolution proposes that compared to other primates, traditional human ecology favored higher levels of investment in both future reproduction and quality of offspring. It is useful to think of short- and long-term responses to various environments and, hence, various optimal allocation regimes. Natural selection can favor the evolution of physiological and psychological mechanisms that facilitate short-term adjustments to environmental variation. The degree of phenotypic plasticity that evolves will represent a compromise between the costs and benefits of flexible responses and also reflect the range of environmental variation experienced by the organism. Humans clearly demonstrate a high degree of adaptive flexibility, mediated through both physiology and behavior. Although the mechanisms underlying our response system evolved in the context of a hunting and gathering way of life, this evolved flexibility is apparent in our recent history as well. Changes in investments in income-related capital, mortality reduction, and maintenance associated with the demographic transition may reflect increased returns to those investments, stimulated by the increasing importance of skills-based competitive labor markets. Similarly, within developed countries, those that have more to gain from investments in education also invest more in longevity and health. Long-term adjustments occur when one short-term response system is competitively more effective than another response system. In general, this would occur when environments change sufficiently so that the ancestral response system produces unfavorable outcomes. We cannot expect natural selection to have
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altered our response system in relation to the contingencies of modern environments, given their very recent appearance. With respect to contemporary issues, such as aging, fertility, and development, the fundamental theoretical issue faced by the social, behavioral, and medical sciences is how to build models of an ancient, but flexible, response system in a very novel environment. References Alexander, R.D. 1974 The evolution of social behavior. Annual Review of Ecology and Systematics 5:325-383. Altmann. J. 1980 Baboon Mothers and Infants. Cambridge, MA: Harvard University Press. Austad, S.N., and K.E. Fischer 1991 Mammalian aging, metabolism, and ecology: Evidence from the bats and marsupials. Journal of Gerontology B47-B53. 1992 Primate longevity: Its place in the mammalian scheme. American Journal of Primatology 28:251-61. 1993 Retarded senescence in an insular population of Virginia opossums (Didelphis virginiana) . Journal of Zoology (London) 229:695-708. Becker, G.S. 1975 Human Capital, 2nd ed. New York: Columbia University Press. 1991 A Treatise on the Family Enl. ed. Cambridge, MA: Harvard University Press. Becker, G.S., and R.J. Barro 1988 A reformulation of the economic theory of fertility. Quarterly Journal of Economics 103(1):1-25. Ben-Porath, Y. 1967 The production of human capital and the life cycle of earnings. Journal of Political Economy 75:352-365. Blurton Jones, N. 1986 Bushman birth spacing: a test for optimal interbirth intervals. Ethology and Sociobiology 4:145-147. 1987 Bushman birth spacing: Direct tests of some simple predictions. Ethology and Sociobiology 8:183-203. 1993 The lives of hunter-gatherer children: Effects of parental behavior and parental reproductive strategy. Pp. 309-326 in M.E. Pereira and L.A. Fairbanks, eds., Juvenile Primates. Oxford: Oxford University Press. Blurton Jones. N., K. Hawkes, and J. O'Connell 1989 Modeling and measuring the costs of children in two foraging societies. Pp. 367-390 in V. Standen and R.A. Foley, eds., Comparative Socioecology: The Behavioral Ecology of Humans and Other Mammals. London: Basil Blackwell. In press Why do Hadza children forage? In N.L. Segal, G.E. Weisfeld, and C.C. Weisfeld, eds., Genetic. Ethological and Evolutionary Perspectives on Human Development. Essays in honor of Dr. Daniel G. Freedman. American Psychological Society. Blurton Jones, N., K. Hawkes, and P. Draper 1994a Foraging returns of !Kung adults and children: Why didn't !Kung children forage? Journal of Anthropological Research 50(3):217-248. 1994b Differences between Hadza and !Kung children's work: Original affluence or practical reason? Pp. 189-215 in E.S. Burch, Jr. and L.J. Ellanna, eds., Key Issues in Hunter-Gatherer Research. Oxford: Berg.
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Blurton Jones, N., and M. Konner 1976 !Kung knowledge of animal behavior. Pp. 325-348 in R.B. Lee and 1. DeVorc. eds., Kalahari Hunter-Gatherers. Cambridge, MA: Harvard University Press. Bock, J. 1995 The Determinants of Variation in Children's Activities in a Southern African Community. Ph.D. dissertation. University of New Mexico. Caro, T.M., D. Sellen, A. Parish. R. Frank, D. Brown, E. Voland. and M. Borgerhoff Mulder 1995 Termination of reproduction in nonhuman and human female primates. International Journal of Primatology 16:205-220. Chagnon, N. 1974 Studying the Yanomamo. New York: Holt, Rinehart and Winston. 1983 Yanomamo: The Fierce People, 3rd ed. New York: Holt, Rinehart and Winston. 1988 Life histories, blood revenge, and warfare in a tribal population. Science 239:985-992. Chagnon, N., and R. Hames 1979 Protein deficiency and tribal warfare in Amazonia: New data. Science 20(3):910-913. Charlesworth, B. 1994 Evolution in Age-Structured Populations, 2nd ed. Cambridge: Cambridge University Press. Chamov. E.L. 1993 Life History Invariants. Oxford: Oxford University Press. Courtenay, J., and G. Santow 1989 Mortality of wild and captive chimpanzees. Folia Primatologica 52:167-177. Divale, W.. and M. Harris 1976 Population, warfare and the male supremacist complex. American Anthropologist 78:521-538. Draper, P. 1975 !Kung women: Contrasts in sexual egalitarianism in foraging and sedentary contexts. Pp. 77-109 in R. Reiter, ed., Toward an Anthropology of Women. New York: Monthly Review Press. 1976 Social and economic constraints on child life among the !Kung. Pp. 199-217 in R.B. Lee and 1. DeVore, eds., Kalahari Hunters and Gatherers. Cambridge, MA: Harvard University Press. Draper, P., and E. Cashdan 1988 Technological change and child behavior among the !Kung. Ethnology 27:339-365, Ellison, P. 1990 Human ovarian function and reproductive ecology: New hypotheses. American Anthropologist 92:933-954. 1995 Understanding natural variation in human ovarian function. In R.I.M. Dunbar, ed., Human Reproductive Decisions: Biological and Social Perspectives. London: MacMillan. Ellison, P.T., N.R. Peacock, and C. Lager 1989 Ecology and ovarian function among Lese women of the Ituri forest. American Journal of Physical Anthropology 78:519-526. Finch, C.E. 1994 The evolution of ovarian oocyte decline with aging and possible relationships to Down syndrome and Alzheimer disease. Experimental Gerontology 29:299-304. 1996 Biological bases for plasticity during aging of individual life histories. D. Magnusson. Pp. 488-512 in The Life span Development of Individuals: Biological and Psychosocial Perspectives, a Synthesis. Cambridge: Cambridge University Press. Finch, C.E., and J.F. Nelson 1994 Genotypic influences on reproduction and reproductive aging in mice: Influences from alleles in the H-2 complex and other loci. Pp. 93-108 in D.K. Sarkar and C.D. Barnes,
OCR for page 206
eds., The Reproductive Neuroendocrinology of Aging and Drug Abuse. Boca Raton, FL: CRC Press. Finch, C.E., and M.C. Pike 1996 Maximum life span predictions from the Gompertz mortality model. Journal of Gerontology 51A(3):B183-B194. Finch, C.E., M.C. Pike, and M. Witten 1990 Slow mortality rate accelerations during aging in animals approximate that of humans. Science 249:902-905. Gadgil, M., and W.H. Bossert 1970 Life historical consequences of natural selection. American Naturalist 104:1-24. Gaulin. S. 1980 Sexual dimorphism in the human postreproductive life span: Possible causes. Human Evolution 9:227-232. Goodall, J. 1986 Chimpanzees of Gombe: Patterns of Behavior. Cambridge, MA: Harvard University Press. Gould, K.G. M. Flint. and C.E. Graham 1981 Chimpanzee reproductive senescence: A possible model for the evolution of menopause. Maturitus 3:157-166. Hames, R. 1983 The settlement pattern of a Yanomamo population bloc: A behavioral ecological interpretation. Pp. 393-427 in R.B. Hames and W.T. Vickers, eds., Adaptive Responses of Native Amazonians. New York: Academic Press. 1992 Variation in paternal investment among the Yanomamo. Pp. 85-110 in B. Hewlett, ed., Father-Child Relations: Cultural and Biosocial Contexts. Chicago: Aldine de Gruyter. Hamilton, W.D. 1966 The moulding of senescence by natural selection. Journal of Theoretical Biology 12:12-45. Harcourt, A.H., and D. Fossey 1981 Demography of Gorilla gorilla. Journal of Zoology. (London) 195:215-233. Harpending, H., and L. Wandsnider 1982 Population structure of Ghanzi and Ngamiland !Kung. Current Developments in Anthropological Genetics 2:29-50. Harris, M. 1977 Cannibals and Kings, New York: Random House. Hawkes, K., H. Kaplan, K. Hill, and A.M. Hurtado 1987 Aché at the settlement: Contrast between farming and foraging. Human Ecology 15(2): 133-161. Hawkes, K., K. Hill. and J. O'Connell 1982 Why hunters gather: Optimal foraging and the Aché of eastern Paraguay. American Ethnologist 9:379-398. Hawkes, K., J. O'Connell, and N. Blurton Jones 1989 Hardworking Hadza grandmothers. Pp. 341-366 in V. Standen and R.A. Foley, eds., Comparative Socioecology: The Behavioral Ecology of Humans and Other Mammals. London: Basil Blackwell. 1991 Hunting income patterns among the Hadza: Big game. common goods, foraging goals and the evolution of the human diet. Pp. 243-251 in A. Whiten and E. Widdowson. eds., Foraging Strategies and Natural Diet of Monkeys, Apes and Humans. Proceedings of the Royal Society of London 334. Oxford: Clarendon Press. 1995 Hadza children's foraging: Juvenile dependency, social arrangements and mobility among hunter-gatherers. Current Anthropology 36:688-700.
OCR for page 207
1996 Hadza Women's Time Allocation, Offspring Provisioning, and the Evolution of Long Post-Menopausal Life spans. Unpublished manuscript. Department of Anthropology, University of Utah, Salt Lake City, UT 84112. Hill, K. 1993 Life history theory and evolutionary anthropology. Evolutionary Anthropology 2(3):78-88. Hill, K., and K. Hawkes 1983 Neotropical hunting among the Aché of eastern Paraguay. Pp. 139-188 in R. Hames and W. Vickers, eds., Adaptive Responses of Native Amazonians. New York: Academic Press. Hill, K.. and A.M. Hurtado 1991 The evolution of reproductive senescence and menopause in human females. Human Nature 2(4):315-350. 1996 Aché Life History: The Ecology and Demography of a Foraging People. New York: Aldine de Gruyter. Hill, K., and H. Kaplan 1988a Tradeoffs in male and female reproductive strategies among the Ache. part 1. Pp. 277-290 in L. Betzig, P. Turke, and M. Borgerhoff Mulder, eds., Human Reproductive Behavior. Cambridge: Cambridge University Press. Hill, K., and H. Kaplan 1988b Tradeoffs in male and female reproductive strategies among the Aché. part 2. Pp. 291-306, in. L. Betzig, P. Turke, and M. Borgerhoff Mulder, eds., Human Reproductive Behavior. Cambridge: Cambridge University Press. Hill, K., H. Kaplan, K. Hawkes, and A.M. Hurtado 1985 Men's time allocation to subsistence work among the Aché of eastern Paraguay. Human Ecology 13:29-47. Holliday, M.A. 1978. Body composition and energy needs during growth. Pp. 117-139 in F. Falker and J. M. Tanner, eds., Human Growth. Vol. 2. New York: Plenum. Howell, N. 1979 Demography of the Dobe !Kung. New York: Academic Press. Huffman, S. L., A.M.K. Chowdhury, J. Chakborty, and W.H. Mosley 1978 Nutrition and postpartum amenorrhea in rural Bangladesh. Population Studies 32:251-260. Hurtado, A. M. 1985 Women's Subsistence Strategies among Aché Hunter-Gatherers of Eastern Paraguay. Ph.D. dissertation, University of Utah. Hurtado, A.. K. Hawkes, K. Hill, and H. Kaplan 1985 Female subsistence strategies among Aché hunter-gatherers of eastern Paraguay. Human Ecology 13:1-28 Hurtado, A.. and K. Hill 1987 Early dry season subsistence ecology of the Cuiva foragers of Venezuela. 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. Journal of Anthropological Research 46:293-345. 1992 Paternal effect on offspring survivorship among Aché and Hiwi hunter-gatherers: Implications for modeling pair-bong stability . Pp. 31-55 in B. Hewlett. ed., Father-Child Relations: Cultural and Biosocial Contexts. New York: Aldine de Gruyter. Hurtado, A., K. Hill, H. Kaplan, and I. Hurtado 1992 Tradeoffs between female food acquisition and child care among Hiwi and Ache foragers. Human Nature 3(3): 185-216.
OCR for page 208
Kaplan, H. 1994 Evolutionary and wealth flows theories of fertility: Empirical tests and new models. Population and Development Review 20(4):753-791. 1996 A theory of fertility and parental investment in traditional and modern human societies. Yearbook of Physical Anthropology 39:91 - 135. Kaplan, H., and K. Hill 1985 Food sharing among Aché foragers: Tests of explanatory hypotheses. Current Anthropology 26(2):223-245. Kaplan, H., J.B. Lancaster, J.A. Bock, and S.E. Johnson 1995 Does observed fertility maximize fitness among New Mexican men? A test of an optimality model and a new theory of parental investment in the embodied capital of offspring. Human Nature 6:325-360. Kelly. R.L. 1995 The Foraging Spectrum. Washington, DC: Smithsonian Institution Press. Kirkwood, T.B.L. 1981 Repair and its evolution: Survival versus reproduction. Pp. 165-189 in C.R. Townsend and P. Calow, eds., Physiological Ecology: An Evolutionary Approach to Resource Use. Oxford: Blackwell. Kirkwood, T.B.L., and M.R. Rose 1991 Evolution of senescence: Late survival sacrificed for reproduction. Pp. 15-24 in P.H. Harvey, L. Partridge, and T.R.E. Southwood, eds., The Evolution of Reproductive Strategies. Cambridge: Cambridge University Press. Konner. M., and M. Shostak 1987 Timing and management of birth among the !Kung: Biocultural interaction in reproductive adaptation. Current Anthropology 2:11-28. Konner, M., and C. Worthman 1980 Nursing frequency, gonadal function, and birth spacing among !Kung hunter-gatherers. Science 207:788-791. Kozlowski, J., and R.G. Weigert 1986 Optimal allocation of energy to growth and reproduction. Theoretical Population Biology 29:16-37. Lancaster, J.B. 1986 Human adolescence and reproduction: An evolutionary perspective. Pp. 17-38 in J.B. Lancaster and B.A. Hamburg, eds., School-Age Pregnancy and Parenthood. New York: Aldine de Gruyter. Lancaster, J.B., and B. King 1992 An evolutionary perspective on menopause. Pp. 7-15 in V. Kerns and J. Brown, eds., In Her Prime: New Views of Middle-Aged Women. Chicago: University of Illinois Press. Lee, R.B. 1979 The !Kung San: Men. Women and Work in a Foraging Society. Cambridge: Cambridge University Press. 1984 The Dobe !Kung. New York: Rinehart and Winston. 1985 Work, sexuality, and aging among !Kung women. Pp. 23-35 in J. Brown and V. Kerns, eds., In Her Prime: A New View of Middle-Aged Women. South Hadley, MA: Bergin and Garvey. Lee, R.B., and I. DeVore, eds. 1976 Kalahari Hunter-Gatherers: Regional Studies of the !Kung San and Their Neighbors. Cambridge. MA: Harvard University Press. Leslie, P.W., and P.H. Fry 1989 Extreme seasonality of births among nomadic Turkana pastoralists. American Journal of Physical Anthropology 79: 103-115.
OCR for page 209
Lunn, P.. S. Austin, A.M. Prentice, and R. Whitehead 1984 The effect of improved nutrition on plasma prolactin concentrations and postpartum infertility in lactating Gambian women. American Journal of Clinical Nutrition 39:227-235. Melancon, T. 1982 Marriage and Reproduction among the Yanomamo Indians of Venezuela. Ph.D. dissertation, Pennsylvania State University. Mincer, J. 1974 Schooling, Experience and Earnings. Chicago: National Bureau of Economic Research. Oates, J.F. 1987 Food distribution and foraging behavior. Pp. 197-209 in B.B. Smuts, D.L. Cheney. R.M. Seyfarth, R.W. Wrangham, and T.T. Struhsaker, eds., Primate Societies. Chicago: University of Chicago Press. O'Connell, J.F., and K. Hawkes 1981 Alyawara plant use and optimal foraging theory. Pp. 99-125 in E. Smith and B. Winterhalder, eds., Hunter-Gatherer Foraging Strategies. Chicago: Chicago University Press. O'Connell, J.F., K. Hawkes, and N. Blurton Jones 1988 Hadza scavenging: Implications for Plio-Pleistocene hominid subsistence. Current Anthropology 29:356-363. O'Connell, J., P. Latz, and P. Barnett 1983 Traditional and modern plant use among the Alyawara of central Australia. Economic Botany 37:80-109. Peacock, N.R. 1985 Time Allocation, Work and Fertility Among Efe Pygmy Women in the Ituri Forest of Northeast Zaire. Ph.D. dissertation, Harvard University. Pennington, R., and H. Harpending 1988 Fitness and fertility among Kalahari !Kung. American Journal of Physical Anthropology 77:303-319. Pope, S.K., L. Whiteside, J. Brooks-Gunn, K. Kelleher, V. Rickert, R. Bradley, and P. Casey 1993 Low-birth-weight infants born to adolescent mothers. Journal of the American Medical Association 269:1396-1400. Prentice, A., and R. Whitehead 1987 The energetics of human reproduction. Symposia Zoological Society of London 57:275-304. Rebuffe-Scrive, M., L. Enk, N. Crona, P. Lonnroth, L. Abrahamsson, U. Smith, and P. Bjorntorp 1985 Fat cell metabolism in different regions in women-effect of menstrual cycle, pregnancy and lactation. Journal of Clinical Investigation 75:1973-1976. Richardson, S.J., V. Senikas, and J.F. Nelson 1987 Follicular depletion during the menopausal transition: Evidence for accelerated loss and ultimate exhaustion. Journal of Clinical Endocrinology and Metabolism 65(6):1231-1237. Rogers, A. 1990 The evolutionary economics of human reproduction. Ethology and Sociobiology 11:479-495. 1993 Why menopause? Evolutionary Ecology 7:406-4200. 1994 Evolution of time preference by natural selection. American Economic Review 84(3):460-481. Rogers, A., and N. Blurton Jones 1992 Allocation of Parental Care. Unpublished manuscript. Department of Anthropology. University of Utah, Salt Lake City, UT 84112.
OCR for page 210
Schrire, C. 1980 An inquiry into the evolutionary status and apparent identity of San hunter-gatherers. Human Ecology 8:9-32. Short, R.V. 1984 Breast feeding. Scientific American 250:35-41. Silk, J.B. 1978 Patterns of food-sharing among mother and infant chimpanzees at Gombe National Park, Tanzania. Folia Primatologica 29:129-141. 1979 Feeding, foraging. and food sharing behavior in immature chimpanzees. Folia Primatologica 31:12-42. Simms, S. 1984 Aboriginal Great Basin Foraging Strategies: An Evolutionary Approach. Ph.D. dissertation. University of Utah. Smith, C.C., and S.D. Fretwell 1974 The optimal balance between size and number of offspring. American Naturalist 108:499-506. Stearns, S. 1992 The Evolution of Life Histories. Oxford: Oxford University Press. Steward. J.H. 1938 Basin Plateau Aboriginal Sociopolitical Groups. Bureau of American Ethnology Bulletin 120. Washington, DC. Teleki. G. 1973 The Predatory Behavior of Wild Chimpanzees. Lewisburg, PA: Bucknell University Press. Terborgh, J.W. 1983 Five New World Primates: A Study in Comparative Ecology, Princeton. NJ: Princeton University Press. Trivers, R. L. 1972 Parental investment and sexual selection. Pp. 136-179 in B. Campbell, ed., Sexual Selection and the Descent of Man. Chicago: Aldine. U.S. Bureau of the Census 1985 Money income of households, families and persons in the United States, 1985. Current Population Reports. Series P-60, No. 156, Table 35. Washington. DC: U.S. Department of Commerce. vom Saal. F.S.. C.E. Finch, and J.F. Nelson 1994 Natural history and mechanisms of reproductive aging in humans, laboratory rodents, and other selected vertebrates. Pp. 1213-1314 in E. Knobil and J.D. Neill, eds., The Physiology of Reproduction, 2nd ed. New York: Raven Press. Wade, G.. and J. Schneider 1992 Metabolic fuels and reproduction in female mammals. Neuroscience and Biobehavioral Reviews 16:235-272. Washburn, S. L. 1981 Longevity in primates. Pp. 11-29 in J. March and J. McGaugh, eds., Aging. Biology and Behavior. New York: Academic Press. Weiss, K. M. 1981 Evolutionary perspectives on human aging. Pp. 25-58 in P. Amoss and S. Harrell, eds., Other Ways of Growing Old. Stanford, CA: Stanford University Press. Wiessner, P. 1982a Measuring the impact of social ties on nutritional status among the !Kung San. Social Science Information 20:641-678.
OCR for page 211
Wiessner, P. 1982b Risk, reciprocity, and social influences on !Kung San economics. Pp. 61-84 in E. Leacock and R.B. Lee, eds., Politics and History in Band Societies. Cambridge: Cambridge University Press. Williams, G. C. 1957 Pleiotropy, natural selection and the evolution of senescence. Evolution 11:398-411. Willis, R.J. 1973 A new approach to the economic theory of fertility behavior. Journal of Political Economy 81:S14-S64. 1987 Wage determinants: A survey and reinterpretation of human capital earnings functions. Pp. 525-602 in O. Ashenfelter and R. Layard, eds., Handbook of Labor Economics. Amsterdam: North Holland. Wilmsen, E.N. 1978 Seasonal effects of dietary intake on Kalahari San. Federation of American Societies or Experimental Biology. Proceedings 37:65-72. 1989 Land Filled with Flies. Chicago: University of Chicago Press. Wood. J. 1994 Dynamics of Human Reproduction: Biology, Biometry, and Demography. New York: Aldine de Gruyter. Woodburn, J.C. 1968 An introduction to Hadza ecology. Pp. 49-55 in R.B. Lee and I. DeVore, eds., Man the Hunter. Chicago: Aldine de Gruyter. 1972 Ecology, nomadic movement and the composition of the local group among hunters and gatherers: An east African example and its implications. Pp. 193-206 in P.J. Ucko, R. Tringham. and G.W. Dimbleby, eds., Man. Settlement and Urbanism. London: Duckworth. 1979 Minimal politics: The political organization of the Hadza of north Tanzania. Pp. 244-266 in W. Snack and P. Cohen, eds., Politics and Leadership: A Comparative Perspective. Oxford: Clarendon Press Wrangham, R.W. 1974 Artificial feeding of chimpanzees and baboons in their natural habitat. Animal Behavior 22:83-94. 1977 Feeding behavior of chimpanzees in Gombe National Park. Tanzania. Pp. 504-537 in H. Clutton-Brock, ed., Primate Ecology. New York: Academic Press. Wrangham, R.W.. and B.B. Smuts 1980 Sex differences in the behavioral ecology of chimpanzees in the Gombe National Park. Tanzania. Journal of Reproductive Fertility Supplement 28:13-31. Yellen, J. 1976 Settlement pattern of the !Kung: An archaeological perspective. Pp. 48-72 in R.B. Lee and 1. DeVore, eds., Kalahari Hunter-Gatherers. Cambridge, MA: Harvard University Press.
Representative terms from entire chapter: