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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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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