The biological aspects of conception and gestation can be discussed in a fairly straightforward way, drawing from what is now known about reproductive biology: A sperm fertilizes an egg; the resulting zygote travels through a uterine (fallopian) tube to become implanted in the uterine lining; and the embryo develops until it’s mature enough to survive outside the womb, at which time birth occurs. But this is clearly not all there is to human pregnancy and birth. Female biology may be similar the world over, but cultural rules and practices are the primary determinants of who will get pregnant as well as when, where, how, and by whom.
Once a pregnancy has begun, there’s much variation in how a woman should behave, what she should eat, where she should and should not go, and how she should interact with other people. Almost every culture known, including our own, imposes dietary restrictions on pregnant women. Many of these appear to serve an important biological function, particularly that of keeping the woman from ingesting toxins that would be dangerous for the fetus. Alcohol is a good example of a potential toxin whose consumption in pregnancy is discouraged in the United States (Fig. 16-12). The food aversions to coffee, alcohol, and other bitter substances that many women experience during pregnancy may be evolved adaptations to protect the embryo from toxins. The nausea of early pregnancy may also function to limit the intake of foods potentially harmful to the embryo at a critical stage of development (Profet, 1988; Williams and Nesse 1991; but see Pike, 2000).
As noted above, there is increasing evidence that what happens during prenatal development has lifelong consequences, many of which are irreversible. Scholars of fetal origins research propose that conditions during pregnancy affect such factors as disease susceptibility and metabolism (Kuzawa, 2008); some go even further to suggest that prenatal factors can affect intelligence and temperament as well (Paul, 2010). With evidence that stress, emotions, and pollution, in addition to food and drink, have effects on a developing fetus, the old adage of “eating for two” during pregnancy can be expanded to “living for two.” Given the effects on metabolism, it may be that pregnancy is the best place on which to focus efforts to curb the worldwide rising obesity rates.
Birth is an event that’s celebrated with ritual in almost every culture that has been studied. In fact, the relatively little fanfare associated with childbirth in the United States is unusual by world standards. Because the risk of death for both mother and child is so great at birth, it’s not surprising that it’s surrounded with ritual significance. Perhaps because of the high risk of death, we tend to think that birth is far more difficult for humans than it is for other mammals. But since almost all primate infants have large heads relative to body size, birth is challenging for many primates
As noted above, the human brain is far less developed at birth than it is in other primates. For a species as dependent on learning for survival as we are, it may be adaptive for most of our brain growth to take place in the presence of environmental stimuli rather than in the relatively unstimulating environment of the uterus. This may be particularly true for a species dependent on language. The language centers of the brain develop in the first 3 years of life, when the brain is undergoing its rapid expansion; these 3 years are considered a critical period for the development of language in the human child.
Infancy, as we noted, is the period of nursing; it typically lasts 2 to 3 years in humans. When we consider how unusual it is for a mother to breast-feed her child for even a year in the United States or Canada, this figure may surprise us. But considering that 2 to 4 years of breast-feeding is the norm for chimpanzees, gorillas, orangutans, and women in foraging societies, most anthropologists conclude that 2 to 3 years was the norm for most humans in the evolutionary past (StuartMacadam and Dettwyler, 1995). Other lines of evidence confirm this pattern, including the lack of other foods that infants could consume until the origin of agriculture and the domestication of milk- producing animals. In fact, if the mother died during childbirth in preagricultural populations, it’s very likely that the child would die also unless there was another woman available who could nurse the child. Jane Goodall has noted that this is also true for chimpanzees: Infants who are orphaned before they are weaned do not usually survive. Even those orphaned after weaning, assuming they survive the trauma, are still emotionally dependent on their mothers and exhibit clinical signs of depression for a few months or years after the mother’s death (Goodall, 1986). Human milk, like that of other primates, is extremely low in fats and protein. Such a low nutrient content is typical for species in which mothers are seldom or never separated from their infants and nurse in short, frequent bouts. Not coincidentally, prolonged, frequent nursing suppresses ovulation in marginally nourished women (Konner and Worthman, 1980), especially when coupled with high activity levels and few calorie reserves (Ellison, 2001). Under these circumstances, breast-feeding can help maintain a 4-year birth interval during which infants have no nutritional competition from siblings. Thus nursing served as a natural birth control mechanism in the evolutionary past, as it does in some populations today.
The fact that infants born today can survive and grow without breast milk is further evidence of our species’ flexibility and how cultural mechanisms have enabled us to transcend some of the biological limitations placed on our ancestors and on other mammalian species. But breast milk also provides important antibodies and microbiota that contribute to infant survival and are not available in formula and other milk substitutes. Throughout the world, breast-fed infants have far greater survival rates than those who aren’t breast-fed or who are weaned too early. The only exception is in societies where scientifically developed milk substitutes are readily available and appropriately used; even then, infants don’t get several important antibodies and other immune factors. The importance of adequate nutrients during this period of rapid brain growth can’t be overestimated. Thus it’s not surprising that there are many cultural practices designed to ensure successful nursing. Furthermore, there is increasing evidence that breast- feeding may be protective against later-life obesity, types 1 and 2 diabetes, and hypertension (Pollard, 2008) as well as childhood obesity (Thompson, 2012). On the other hand, for women in the United States, obesity is associated with a lower duration of breast-feeding (Hauff and Demerath, 2012). As in the case of prenatal care, the public health goal of increasing rates of successful breast-feeding around the world is a worthy one.
Humans have unusually long childhoods and a slowed growth process, reflecting the importance of learning for our species (Bogin, 2006; Thompson and Nelson, 2011). Childhood is the time between weaning and puberty; it is a time when growth in stature is occurring, the brain is completing its growth, and technical and social skills are being acquired. For most other mammals, once weaning has occurred, getting food is left to individual effort. Humans may be unique in the practice of providing food for juveniles (Lancaster and Lancaster, 1983) (Fig. 16-14). In the course of human evolution, it’s possible that provisioning children between weaning and puberty may have doubled or even tripled the number of offspring that survived to adulthood (Table 16.1). This long period of extended child care probably enhanced the time for learning technological and social skills, also contributing to greater survival and reproductive success. Thus the costs of extensive parental care were outweighed in human evolutionary history by the benefits of greater reproductive success. It is during childhood that the roles of fathers, older siblings, grandmothers, and other kin become very significant. While mothers are highly involved with caring for new infants, the socialization and care of other children often fall to other family or community members. Clearly family environment, stress, and other biosocial factors have a major impact on children’s health (Flinn, 1999, 2008) ( see Fig. 16-12).
The major causes of childhood death worldwide today are infectious diseases exacerbated by poor nutrition (Caulfield et al., 2004; Pelletier et al., 1995). Noting how important it is to have sufficient caloric intake in infancy while the brain is developing, Christopher Eppig and his colleagues have suggested that infectious diseases and parasitic infections drain nutrients and energy necessary for brain development in ways that threaten cognitive development and may help to explain the variation in IQ scores seen across nations (Eppig et al., 2010). It’s notable that the leading causes of childhood death in the United States and western Europe aren’t typically related to malnutrition; for children under 5 years of age, accidents are the leading cause of death, followed by preterm births.