The fields of human demography and population biology share intellectual roots and a common set of methodological tools for describing and analyzing population processes. The two disciplines, however, have developed independently with very little cross-fertilization. They developed independently probably because human populations experienced very rapid changes in patterns of mortality and fertility from the mid-nineteenth century to the early twenty-first century, suggesting to demographers that explanations of human population processes must be inherently social rather than biological.
Evolutionary demography analyzes population processes as reflecting the optimizing force of natural selection, the process by which alternative genotypes change in frequency due to differential reproduction of the phenotypes with which they are associated in given environments. Thus, gene coding for physiological and psychological mechanisms regulating fertility, mortality, and investment in offspring are seen to evolve under the influence of natural selection. Even though gene frequencies in populations are expected to change rather slowly over generational time, this does not imply similarly slow rates of change in demographic outcomes. Both plants and animals are capable of very rapid and adaptive (i.e, fitness-enhancing) adjustments in vital rates. Thus, rapid changes in human fertility and mortality per se do not imply a major discontinuity between humans and other organisms that would require a completely independent explanatory framework.
This article presents a broad overview of evolutionary demography, with a specific focus on humans. It considers three themes: (1) the timing of life events, including development, reproduction, aging, and risks of mortality; (2) the regulation of reproductive rates and parental investment in offspring; and (3) sexual dimorphism and its relationship to mating and marriage patterns.
Human Life History Adaptation in Comparative Perspective
Humans lived as hunter-gatherers for the vast majority of their evolutionary history (the genus Homo has existed for about 2 million years). Some life history features can be determined from the fossil record, but it is not yet possible to estimate many vital statistics from paleontological and archaeological remains. Modern hunter-gatherers are not living replicas of humans's Stone Age past because global socioeconomic forces affect them all. Yet, in spite of the variable historical, ecological, and political conditions affecting them, there is remarkable similarity among foraging peoples, and even the variation often makes adaptive sense. Comparisons between foraging peoples and other modern primates are an important source of information about the life histories of human ancestors and the selection pressures acting upon them.
Relative to other mammalian orders, the primate order is slow-growing, slow-reproducing, long-lived, and large-brained. Humans are at the extreme of the primate continuum. Figure 1 illustrates the differences between human foragers and wild-living chimpanzees. The age-specific mortality profile among chimpanzees is relatively V-shaped, decreasing rapidly after infancy to its lowest point (about 3% per year) at about 13, the age of first reproduction for females, increasing sharply thereafter. In contrast, mortality among human foragers decreases to a much lower point (about 0.5% per year) and remains low with no increase between about 15 and 40 years of age. Mortality then increases slowly, until there is a very rapid rise beginning around age 70. The pattern is much more "block U-shaped." The strong similarities in the mortality profiles of the foraging populations suggest that this pattern is an evolved life history characteristic of the human species.
As a result of these differences in mortality patterns, hunter-gatherer children experience higher survival rates than chimpanzees to age of first reproduction: about 60 percent to age 19 versus 35 percent to age 13. Chimpanzees also have a much shorter adult lifespan than humans. At first reproduction, chimpanzee life expectancy is an additional 15 years, compared to 38 more years among human foragers. Importantly, women spend more than a third of their adult lives in a postreproductive phase, whereas very few chimpanzee females survive to reach this phase. Fewer than 10 percent of chimpanzees survive to age 40, but some 15 percent of hunter-gatherers survive to age 70.
Age profiles of net food production (food produced minus food consumed) also differ sharply (see Figure 1). Among chimpanzees, net production before age five is negative, representing complete, then partial, dependence upon mother's milk. The second phase is independent juvenile growth, lasting until adulthood, during which net production is zero. The third phase is reproductive, during which females, but not males, produce a surplus of calories that they allocate to nursing. Humans, in contrast, produce less than they consume for some 15 to 22 years, depending on the group. Net production becomes increasingly negative until about age 14 and then begins to climb. Net production of adult humans is much higher than in chimpanzees and peaks at about 35 to 45 years of age. This peak is about five times as high as the chimpanzee peak. The human age profile of production could not exist if humans had the same mortality profile as chimpanzees. Only 30 percent of chimpanzees reach the age when humans produce what they consume on average, and
fewer than 10 percent reach the age when human production peaks.
High levels of knowledge and skill are needed to acquire the variety of high-quality resources humans consume. These abilities require a large brain and a long time commitment to physical and psychological development. This extended learning phase during which productivity is low is compensated for by higher productivity during the adult period. Because productivity increases with age, and therefore the return on the investment in the development of offspring occurs at an older age, the time investment in skill acquisition and knowledge leads to selection for lowered mortality rates and greater longevity. Thus, it is likely that the long human lifespan co-evolved with the lengthening of the juvenile period, increased brain capacities for information processing and storage, and intergenerational resource flows.
Regulation of Reproduction under Natural Fertility Regimes
Traditionally, demographers have attempted to understand the onset and termination of reproduction and birth intervals in natural fertility regimes in terms of proximate determinants (i.e, those that have direct mechanistic impacts on fertility, such as coital and breast-feeding frequencies). For the most part, these determinants are treated as givens and there has been little consideration of the causal processes shaping them. In contrast, evolutionary demographers approach these determinants in terms of design and ask why the physiological, psychological, and cultural processes that regulate fertility take the forms that they do.
There is mounting evidence that human reproductive physiology is particularly specialized toward the production of high-quality, large-brained offspring. Two implications of this specialization are rigid control over embryo quality and a series of adaptations on the part of both mother and offspring designed to ensure an adequate energy supply for the nutrient-hungry, fast-growing brain. Given the massive investment in human offspring, this system ensures that investment is quickly terminated in an offspring of poor genetic quality. Fetal growth is more rapid in humans than in gorillas and chimpanzees, and both mother and offspring store exceptional amounts of fat, probably to support an equally exceptional rate of expensive brain growth during the first five years of life.
The physiological regulation of ovulation, fertilization, implantation, and maintenance of a pregnancy is highly responsive to energy stores in the form of fat, energy balance (calories consumed minus calories expended), and energy flux (rate of energy turnover per unit time). Low body fat, weight loss due to negative energy balance, and extreme energy flux (either very low intake and very low expenditure, or very high intake and very high expenditure) each lower monthly probabilities of conceiving a child that will survive to birth. Seasonal variation in workloads and diet has been shown to affect female fertility. Variation across groups in both age of menarche (first menstrual period) and fertility has been linked to differences in food intake and workload.
Behavior and the underlying psychological processes that govern parental investment in offspring affect fertility indirectly via maternal physiology. One route is through breast-feeding. Patterns of breast-feeding and solid food supplementation vary both cross-culturally and among mother–infant pairs. Unlike the growing body of knowledge about the physiological pathways mediating the effects of nursing, much less is known about the cultural, psychological, and physiological determinants of the duration and intensity of nursing, and the respective roles of the mother and infant in the process.
The second route relates to the additional energetic constraints involved in provisioning children. The age/sex profile of work and productivity, along with a system of food distribution, determine the net energy available for reproduction among women. People in foraging societies are sensitive to ecological variability in the trade-offs regarding children's work effort and their provisioning. Thus, natural selection appears to have acted upon both the psychology of parental investment and maternal physiology to produce a flexible system of fertility regulation. The key to this system is that maximizing lifetime-expected resource production through the optimal allocation of activities and food flows will tend also to maximize fitness when all wealth is in the form of food and when extra food translates into higher fertility. Nevertheless, empirical applications of optimality models, designed to determine whether the onset and termination of reproduction and the size of interbirth intervals actually maximize fitness, have produced mixed results.
Role of Men in Human Reproduction
Unlike most other male mammals, men in foraging societies provide the majority of the energy necessary to support reproduction. Among the ten foraging societies for which quantitative data on adult food production are available, men on average contributed 68 percent of the calories and almost 88 percent of the protein; women acquired the remaining 32 percent of calories and 12 percent of protein. Given that, on average, 31 percent of these calories are apportioned to support adult female consumption, 39 percent to adult male consumption, and 31 percent to consumption of offspring, women supply 3 percent of the calories to offspring and men provide the remaining 97 percent.
Complementarity between the investments of each sex in reproduction appears to be the principal force favoring decreased sexual dimorphism (i.e., differential expression of traits by males and females, respectively) and increased male parental investment. This kind of complementarity can occur when both direct care and resources are important to offspring viability, but they conflict with one another. For example, among many flying bird species, protection and feeding of nestlings are incompatible, leading to biparental investment and turn-taking in feeding and nest protection by males and females.
Hunting, as practiced by humans, is largely incompatible with the evolved commitment among primate females toward intensive mothering, carrying of infants, and lactation-on-demand in service of high infant survival rates. First, it often involves rapid travel and encounters with dangerous prey. Second, it is often most efficiently practiced over relatively long periods of time rather than in short stretches, because of search and travel costs. Third, it is highly skill-intensive, with improvements in return rate occurring over two decades of daily hunting. The first two qualities make hunting a high-cost activity for pregnant and lactating females. The third quality, in interaction with the first and second, generates life course effects such that gathering is a better option for females, even when they are not lactating, and hunting is a better option for males. Because women spend about 75 percent of their time either nursing or more than three months pregnant during their reproductive lives, they never get enough practice to make it worth while to hunt, even when they are not nursing or pregnant, or are postreproductive.
Human females evidence physiological and behavioral adaptations that are consistent with an evolutionary history involving extensive male parental investment. They decrease metabolic rates and store fat during pregnancy, suggesting that they lower work effort and are being provisioned. Women in foraging societies decrease work effort during lactation and focus on high-quality care. In contrast, nonhuman primate females do not store appreciable fat, increase work effort during lactation, and as a result, have increased risk of mortality. The human specialization could not have evolved if women did not depend on men for most of their food provisioning throughout human history.
Extensive cooperation among men and women would make sense only if the reproductive performance of spouses were linked. When women reach menopause in their late forties, men have the option to continue reproducing with younger women but they do not generally do so. Among the Aché, for example, 83 percent of all last births for women also represent a last child for the fathers.
Because men support reproduction only indirectly by affecting the energy intake rates of women and children, it is not surprising that spermatogenesis (the formation of sperm) is buffered from variations in food intake. Natural selection on male physiology and behavior appears to reflect the trade-off between mating investment and survival. Androgens, most notably testosterone, affect muscularity, competitiveness, and high-risk behavior, but they reduce immune function and fat storage, which, in turn, affect survival. Future research on the male endocrine system is likely to elucidate how natural selection has operated to produce both individual variation and age-related changes in male physiology and behavior.
Extra-Somatic Wealth and Its Implications for Human Demography
With the advent of agriculture and pastoralism (live-stock raising), wealth storage in the form of land, livestock, luxury goods, and, eventually, money became commonplace practices. Around the globe, responses to these new economic practices were highly patterned and exhibited remarkable uniformity in response to similar ecological conditions. First, men became actively involved in competition for access to resources, which, in turn, were used for access to women. Second, parental investment extended beyond food and care, and well-defined inheritance practices emerged.
Among pastoralists, the male warfare complex, where livestock are stolen and brides are captured, became common. Patrilineal inheritance (through the male line) of livestock also became the norm, and men gained access to women through the transfer of bride wealth to the women's families. Polygyny (the taking of more than one wife), practiced to a much lesser degree among foragers, resulted from differences in wealth, and the number of wives increased in relation to wealth.
In the case of agriculture, social stratification of wealth increased in relation to the patchiness of arable land, with highly fertile river valleys generating the most intensive competition and the greatest wealth differentials. In a 1993 study of six despotic empires (Mesopotamia, Egypt, Aztec Mexico, Inca Peru, and imperial India and China), Laura Betzig detailed the impressive convergence of cultural evolution. Powerful men sequestered large harems of women (several thousand in the case of rulers) and enforced their exclusive sexual access. Nevertheless, legal marriage–being highly restricted and often monogamous, and controlling inheritance–was differentiated from concubinage. Marriages were often strategic, between families of similar social class, or involving dowries that were exchanged for the upward mobility of women. Inheritance often was distributed differentially with sibships, with the most common pattern being primogeniture. Because the complex societies of North and South America developed in the absence of contact with the Old World, cultural convergences between the New and Old Worlds are likely the result of an evolved human psychology interacting with new technologies of production (which themselves were most likely reactions to demographic pressures and reduced returns from foraging).
Such responses are largely consistent with evolutionary logic and are analogous to the behavior of other organisms, although there is some evidence that human responses to extra-somatic wealth may not be fitness-maximizing. It is possible that the psychology of social status striving, which was perhaps adaptive in foraging societies, is no longer adaptive in the context of extra-somatic wealth.
It is almost certainly the case that reproductive behavior in post–demographic transition societies (that is, those in which small families are the norm and mortality rates are low) is not fitness maximizing. Even taking into account the effects of low fertility on increased parental investment and subsequent adult income of offspring, fertility is much lower than the predicted level for maximizing descendants. For example, a study of men's reproductive behavior in Albuquerque, New Mexico, found that the number of grandchildren was maximized by men with 12 or more children (the highest number reported), even though mean fertility was just over two children.
Nevertheless, just as increased payoffs to skill and mortality reduction may underlie the original evolution of long-term child dependence and human longevity, similar changes may explain the dramatic lowering of fertility accompanying modernization. Education is the best and most consistent predictor of fertility variation, both within and among nations. The payoffs to education have changed for two reasons. Changes in the technology of production within education-based labor markets have led to very high returns on parental investments in children's education. Changing medical technology and improved public health have greatly reduced mortality rates for all age groups. Increased survival rates during the period of parental investment increase the expected costs per child born, favoring further increases in offspring quality. Increased survival during the adult period increases the expected years of return on educational investments, further increasing the incentive to invest in children's education.
In response to the increased payoffs on investments in education and the expected costs of those investments, parents determine the number of children they can afford to rear, given their wealth. These factors result in fertility being regulated by a consciously determined fertility plan realized through birth-control technology and/or controlled exposure to sex. The low mortality rates also allow parents to plan reproduction at the outset, because the number of children born accurately predicts the number of children that will reach adulthood.
A great deal of further research is necessary, however, before there is a full understanding of why these changes in socioecology (i.e., the physical, biotic and social conditions characterizing the environment) have resulted in such low levels of fertility and high levels of parental investment and wealth consumption. People are not simply maximizing family wealth because net wealth of families would be maximized by higher fertility than is currently observed. Nor are they maximizing personal consumption; in that case, they would have no children. One possible hypothesis is that social dynamics of small groups in hunting and gathering economies resulted in greater fitness for those of higher social standing and selected for a psychology in which relative social position of self and offspring is valued highly. This psychology may have been fitness-maximizing under traditional conditions. If relative, as opposed to absolute, wealth and social standing guide human decisions regarding parental investment and fertility, it is possible that "runaway" consumption and investment in children's education result from the interaction of this psychology with modern education-based labor markets and consumption possibilities.
Evolutionary demography is best viewed not as an alternative to traditional approaches but rather as a general theoretical framework that existing models will contribute to and enhance. Economic modeling is fundamental to evolutionary analysis. Economists are primarily concerned with conscious, rational decision processes, but these are only a subset of the regulatory mechanisms of controlling fertility. Feminist demography details the conflicts of interest between men and women, and how they vary with social context. A more basic evolutionary understanding of why men and women differ and how socioecology affects the extent to which the behavior and goals of the two sexes converge and diverge will enrich such insights. Because information about the costs and benefits of alternative behavioral options is often socially acquired, the understanding of cultural diffusion is critical. Evolutionary logic provides a framework for the analysis of the active role that people play in determining which ideas they chose to adopt.
The application of evolutionary theory to humans is complicated by technological and social change. Given that evolution is a historical process, organized, flexible responses to environmental variation evolve when populations are exposed to fluctuations that are patterned and, to some extent, regular. When organisms are exposed to new, and radically different, environments, however, it is possible that their responses may be nonadaptive, reflecting physiological and psychological traits that evolved in the context of more ancient environments. The response of drastically reduced fertility may be a case in point. The analysis of a species in a novel, rapidly changing environment poses special challenges, but an understanding of the evolutionary past of humans should assist in explaining the present and predicting the future.
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