Young infant primates have sufficient motor skills to cling to their mothers soon after birth and can hold on while their mothers forage for food, run to keep up with the group, and sleep at night. The extra weight adds to a mother’s daily food needs, but otherwise, an infant does not appear to interfere a great deal with her ability to carry on with her normal activities. As I noted previously, human infants do not have the degree of motor development necessary to support their own body weight, so they need to be supported by their mothers. Furthermore, their feet have lost the ability to cling because of adaptations to bipedalism, and their mothers do not have enough body hair for the babies to cling to even if they had strong muscles and prehensile toes. Because of the low nutrient density of their milk, the mothers do not have the option of leaving the infants in a nest or in the care of others unless those others are also lactating. They must carry the babies with them throughout the day, either in their arms or in a sling. If the mother had to use her arms to hold her infant, it would limit her ability to carry food or to use tools to acquire foods such as roots and tubers. On the occasions when she did have to put the baby down, she needed a way of keeping the infant quiet to avoid detection by predators. In the view of paleoanthropologist Dean Falk, the need for a mother to soothe the infant with her voice is the foundation of human language.
The Costs of Carrying Infants
Given the costs and inconvenience of carrying infants, the ‘‘invention’’ of the infant sling would have been an important innovation in human evolution and one that allowed women to continue collecting foods even when they had young and helpless infants. In an experimental study, it was demonstrated that it is energetically more costly for a mother to carry her infant in her arms than in a sling.2 Furthermore, the researchers were able to demonstrate that the costs of carrying infants for a female with a narrow pelvis, characteristic of the genus Homo in contrast to the genus Australopithecus, are even greater, placing more selective pressure on making tools for carrying infants as pelvic width narrowed in the course of human evolution. The costs of carrying infants may have been one of the primary components maintaining birth intervals in ancestral humans. Anthropological studies confirm that women travel great distances carrying food even when they are encumbered with young children. Consider that if a woman has an infant every two years,she would have to carry two infants over great distances, at considerable cost to her own energetic resources. Table 8.1 summarizes data that anthropologist Richard Lee collected in his studies of the !Kung, showing the average weight of children at various ages, average distance that a woman walks in a year, and the cost of carrying infants of various ages for each of those kilometres. From this table it can be easily seen how costly it would be to have two children under age 4. And in fact, given the close association between energy availability and ovulation, it is unlikely that a forager woman could accumulate the reserves to ovulate until her infant approached age 3 and was able to walk part of the distance with his mother. For example, if an infant is born when his sibling is 2, the workload for the mother would increase to as much as 35,280 kilograms for a year of carrying the infant and the 2-year-old child. Lee argues that the two-year birth interval would be the theoretical limit to the ability of a woman to meet her own and her infants’ energetic needs. At this rate, she may not be able to support both children, suggesting another point where trade-offs result in longer birth intervals if they are associated with healthier offspring. Of course, if she can get help from other group members (her mate, older children, or older, non reproductive females), she would be able to maintain a lower birth interval.
Human infants need to be carried, but at some point, children begin moving about on their own.Locomotors skills are among the ‘‘developmental milestones’’ that parents and health care providers pay attention to in the first year or two of an infant’s life, and crawling as an expected milestone for a child between 7 and 12 months of age appears on almost all lists. In fact, it is regarded as a universal characteristic of infancy, and parents often become concerned if their child is not crawling by his first birthday. Crawling is seen as an important stage toward full bipedal walking and one necessary for the development of strength and motor control. But anthropologist David Tracer argues that infant crawling is a somewhat recent phenomenon and may well be associated with the appearance of the first floors in houses and hygienic practices that reduced exposure to parasites and predators.4 Consider that an infant who is placed on his belly on the ground is exposed to a lot of noxious agents including human wastes, parasites, hot fires, and various pathogens that can cause diarrhoea and other, potentially worse, diseases. What Tracer found to be more common in many indigenous cultures, including the Au people that he studied in Papua New Guinea, was carrying for a longer period of time and placing the child in an upright, seated position when he was awake but not being carried. The phase that precedes walking in this population is upright ‘‘scooting.’’ In fact, crawling is actively discouraged by Au parents. As a consequence, Au infants ‘‘fail’’ the horizontal tests in the widely used Bayley Scales of Motor Development, but do quite well in the vertical positions, the opposite of what is seen (and judged as ‘‘normal’’) in Western populations. Yet again we have evidence that what is normal in one culture may be quite the opposite in another.
Sleeping with Infants
As noted, constant contact includes sleeping with the infant and breastfeeding throughout the night, certainly a component of reproductive strategy for humans and most other primates. It is part of the human evolutionary legacy that infants ‘‘expect’’ to be with their mothers at all times. Consider what would have happened to an infant in the evolutionary past who was put down in another part of the sleeping area while the parents tried to ignore the cries throughout the night. It makes sense from every possible angle to argue that mothers and infants slept together throughout history, just as they do in most traditional cultures today. But the American Consumer Product Safety Commission, well-meaning paediatricians, and the general public in the United States have different views. Many health care professionals and parents consider that sleeping with your baby can be dangerous and can lead to a myriad of disorders from death (due to sudden infant death syndrome/SIDS or parents rolling over and crushing the baby) to poor psychosexual development. Anthropologists Jim McKenna and Thom McDade suggest that rather than being a source of night-time comfort and sustenance, the mother’s body is often seen as a ‘‘lethal weapon.’’5 An evolutionary perspective presents a different view. Jim McKenna is probably the best-known advocate of what he refers to as the ‘‘biologically appropriate sleeping arrangement’’ for human mothers and infants. 6 In fact, it should be obvious that night-time breastfeeding is nearly impossible if the infant and mother are not in close proximity. As noted earlier, ancestral infants who were put aside during the night were very likely not to survive until morning due to cold, predation, and hunger. So if breastfeeding throughout the day and night is part of the human legacy, then so is co-sleeping. Furthermore, despite all sorts of negative press, mothers in the United States choose to sleep with or very near their infants in huge numbers, especially if they are also breastfeeding. When they do sleep with their infants, mothers report that the infants cry less, both sleep better, and she produces more milk.McKenna’s research shows that mothers are more responsive to infant arousals and would be more likely to detect breathing problems that the infant might experience, thus preventing rather than causing SIDS.8 In his view, babies are more likely to die from not sleeping with their parents than when sleeping with them, except in cases where a parent is under the influence of alcohol or drugs. He believes that in most instances, SIDS is a culturally influenced cause of death that is rarely associated with biology. Although McKenna’s research does not allow an actual recommendation of bed sharing, it does reject universal recommendations against the practice.
As discussed earlier, one of the most frequent complaints of new parents is inability to get sufficient sleep when a baby joins the household. McKenna notes that the phrase ‘‘good baby’’ is ‘‘practically synonymous with a baby’s ability to ‘sleep through the night’ alone.’’ In fact, a question about sleep is one that is frequently asked of a new mother by paediatricians and friends in the United States, indicating an almost universal expectation that sleeping through the night alone is a milestone to be achieved as soon as possible. It is also clear that early independence is a behaviour encouraged by Western societies like that of the United States, and solitary sleep serves to implement that goal. McKenna notes that the solitary-sleeping infant who quickly‘‘learns’’ to sleep through the night is both the valued and the‘‘normal’’infant, although it is a far cry from human infants of millions of years of evolutionary history. The evolutionary perspective has successfully argued that feeding human infants on a four-hour schedule not only does not make sense but is unrelated to mother and infant biology. Gradually this perspective is also changing the way parents and pediatricians view infant sleep. Obviously,there is a simple and time-honored solution to the ‘‘problem’’ of infants not sleeping through the night. Other aspects of infant sleep in addition to location can benefit from examination through an evolutionary lens. For example, there is great concern about the dangers that result from infants being placed in bed on their stomachs. In fact, the Back to Sleep campaign was initiated in 1992 by a coalition of medical organizations in an effort to quickly educate caretakers about how to place their infants in beds, based on findings that infants who were placed on their stomachs were at higher risk of dying from SIDS. It has been quite successful, resulting in a 50% reduction in SIDS deaths since it began. But as McKenna points out, breastfeeding mothers do not need slogans to tell them how to place their babies in bed; babies placed on their stomachs have great difficulty getting in a position to nurse, so a mother who wants to facilitate nursing is much more likely to place her infant on his back. Mothers who are breastfeeding seem to instinctively sleep on their sides with their legs drawn up in a way that partially encompasses the infant and would prevent her from rolling over on her baby. When babies sleep in close proximity to the mother, they exhibit more arousals, more frequent breastfeeding, increased heart rate and body temperature, more movement and awakenings, and less time in deep sleep. In fact, the deep sleep that most of us regard as desirable is far from the ideal for infants in the first few months of life when sleep interruptions (apneas) are frequent and failure to awake easily in response to them can result in suffocation and death.
It has been frequently noted that there is no animal model for SIDS. In other words, this seems to be a disorder unique to humans, perhaps to modern, Western humans who do not sleep with their infants. But the fact that it can occur in human infants and apparently does not occur in other species suggests an underlying anatomical or physiological predisposition. Among the places to look for these are the developing brain and the respiratory anatomy. Between two and five months of age, the time when SID occurrence is most likely, self-regulatory behaviours (like breathing control) are changing from being reflexively to voluntarily controlled. The infant before this time apparently lacks ‘‘natural’’ responses to things that block breathing (such as mucus or bed covers) and must depend on environmental experiences to ‘‘learn how to breathe’’ and maintain breathing passageways. What better environmental cues are there than the regular, rhythmic breathing of the mother and the carbon dioxide that she expels close to the baby’s face? When the infant sleeps alone, those tactile and auditory cues are absent, and breathing irregularities may become lifethreatening. The fact that SIDS is more common (and may be unique) in humans than in other species is likely due to two factors: (1) the neurologically underdeveloped human brain; and (2) cultural factors that lead to infants sleeping alone. Another biological factor influencing SIDS may be the structure of the upper respiratory tract, which is different in humans compared to other mammals. An important difference is that in humans, the breathing (larynx) and the swallowing (pharynx) channels converge rather than remain separate. This means that humans cannot breathe and swallow simultaneously, as other animals can. But because this inability to breathe and swallow at the same time would prevent nursing from occurring, the configuration does not appear until an infant is about 2 years old, about the time that breastfeeding frequency decreases. In other words, the human infant looks like all other mammals with regard to the configuration of the breathing and swallowing passageways. Initial changes in the configuration begin between 4 and 6 months of age, the time when SIDS rates are highest. Anthropologist Jeffrey Laitman noted that this ‘‘may be a time of potential respiratory instability due to a changeover from one respiratory pattern to another.’’ The unique configuration appears to be related to bipedalism and upright posture, which explains why it does not appear until the human infant is a biped. Furthermore, although it seems to cause numerous medical problems (many people die of choking every year), the enlarged region where the two tubes converge allows for the almost-infinite array of sounds associated with human speech. No matter how smart your infant seems to be, he or she will not have full language abilities until that convergence occurs. One of the arguments used by paediatricians for urging that infants sleep alone is that only this will lead to healthy sleeping behaviour later during childhood, adolescence, and adulthood. But not being able to get a good night’s sleep is cited as a problem for 62% of contemporary American adults who were presumably ‘‘taught’’ good sleeping behaviour by being placed alone in their cribs.5 I wonder if the statistics would be different had parents and infants slept together for the last several decades rather than apart. It seems that if sleeping alone as infants were the ‘‘solution’’ we would not see so much evidence of sleeping problems in people of all ages. In fact, it is much more common for parents in the West to report sleep problems in their children than those in cultures where co-sleeping at all ages is common. Finally, by almost any measure (self-esteem, sociability, life satisfaction), co-sleeping has been found to have more positive outcomes than solitary sleeping in studies that have been conducted in the past two decades. As McKenna notes, this is evidence that the benefits of co-sleeping may be as great for 21st-century people as they were for our ancestors. Of course, if the beds in which we 21st-century folks sleep have huge fluffy comforters or duvets, lots of pillows, are waterbeds, or include people who are obese, smoke, or are under the influence of alcohol or drugs, it may not be a good place to sleep now or in the past, no matter the other benefits.
Weaning and Beyond
Weaning is another topic that has been examined through the lens of evolutionary medicine. At some point, the reproductive ‘‘goals’’ of the mother and of her nursing infant come into conflict again when it is in her best interest to wean so that she can reproduce again—but it is in his best interests to continue deriving nutrients and immune factors from her for as long as possible. That point varies across species and across human populations, and there is variation in what may be regarded as the ‘‘normal’’ period of breastfeeding. A lot depends on the age of the mother (and thus her reproductive potential), the amount of pathogens in the environment, and the availability of appropriate foods for newly weaned infants. Anthropologist Dan Sellen proposes that weaning can come younger in humans than in comparative primate species because we have the ability to prepare what he calls ‘‘transitional foods’’ for feeding infants as they are being weaned. Among chimpanzees and most human foraging cultures, three to four years seems to be the typical period of infant nursing. Anthropologist Katherine Dettwyler has examined a number of life history traits to determine what she calls the ‘‘natural age of weaning’’ for humans. For example, larger animals tend to nurse their infants longer relative to gestation length (for gorillas and chimpanzees the ratio of nursing to gestation length is 6:1) which would suggest that the weaning age for humans is 4.5 years. In many species of monkeys and apes, weaning occurs at the time of eruption of the first molars, which would be about 6 years in humans. Dettwyler reports that primates wean their young when they have reached about a third of adult body weight, about five to seven years for humans. For those of us in cultures where breastfeeding, if it occurs at all, often lasts less than one year, these numbers (four to seven years) seem excessive, but for people in parts of the world where access to appropriate and healthful infant foods is limited, nursing for several years (with supplementation from other sources after six months) may mean the difference between a healthy and a sickly childhood.
On the other hand, anthropologist Gail Kennedy argues that the developing human infant brain cannot be supported on mother’s milk alone beyond one year, so we should expect supplementation and even weaning by that time if there are sufficient high-density foods available. Great apes can ‘‘afford’’ to breastfeed their infants for a long time, thus protecting them from malnutrition and infection, because they do not have to support the growth of a hungry, metabolically expensive brain. Humans, however, have a dilemma, according to Kennedy, in that they must sacrifice the immune protection of mother’s milk for higher energy foods to support brain growth. By about 3 years of age, the brain simply cannot continue to develop with mother’s milk alone or even with some supplementation, so safety is sacrificed with a transition to adult foods. If this strategy is the result of natural selection, it suggests that survival alone is not sufficient to ensure human reproductive success; rather, intellectual development and accompanying social and technological skills have equal impact. Of course, 3-year-old infants are not able to secure high-quality adult foods for brain development without extensive assistance and direct provisioning from parents and kin. Kennedy argues that the shift from the ape 5-years-of-nursing pattern to the human 2.5-years-of-nursing pattern occurred with a ‘‘tool-assisted dietary shift’’ about 2.5 million years ago, associated with an increase in consumption of foods of animal origin. Help from kin, especially maternal grandmothers, may have been crucial to the success of this transition.
No matter when it occurs, weaning is far from the end of parental care. As noted, one characteristic that distinguishes humans from other mammals and most other primates is that we continue to provide food for our young following weaning. For most young mammals, once they are weaned, getting food is pretty much up to individual effort. Sharing of food, even between mother and child, is rare in other species. Anthropologists Chet Lancaster and Jane Lancaster suggest that the behaviour of provisioning children between weaning and puberty may have doubled or even tripled the number of offspring that survived to adulthood for early humans. This long period of extended child care by older children and adults probably enhanced the time for learning technological and social skills, also contributing to greater survival and reproductive success. It also provided time for continued brain development, which is mostly finished by age 6. Thus, the costs of extensive parental care may have been outweighed in human evolutionary history by the benefits of greater reproductive success for mothers and for the offspring who were provisioned. Furthermore, Dan Sellen has suggested that the practice of preparing foods for infants as they are being weaned served as both a protection for the infants and as a way of reducing energy costs for the mother, leading to increased reproductive success for both and adding to the behavioural flexibility of human adaptation. He argues that this may account for the lower birth intervals seen in humans in comparison with our closest primate relatives. But if the ‘‘complementary foods’’ are inappropriate, as they often are today, the strategy may backfire, resulting in poor infant and child health. Sellen argues that interventions that target increasing infant and maternal health need to take into account the behavioural flexibility of infant weaning and the tendency to wean too early even when complementary foods are not adequate.
Future Motherhood and Breasts
Getting information out to the general public (especially those who are pregnant or planning to get pregnant) about the benefits of mother’s milk for babies has apparently been so successful that human inventiveness and modern technology have come up with ways that moms can provide their milk to their infants when they are not able to actually breastfeed them. In this way, they can avoid being tied down to a breastfeeding regimen that is incompatible with other aspects of their lives such as working outside the home. Jill Lepore writes in the New Yorker that mechanical breast pumps are almost as ubiquitous as cell phones for mothers of young infants and are even advertised as appropriate gifts for baby showers. For women who must return to work soon after their babies are born, these pumps are welcomed because they enable them to continue to provide what they regard as the best food they can offer their infants. Perhaps ‘‘breast is best,’’ but mother’s milk without the breast is at least better than most alternatives when a woman has no other choice. This is not to say that pumping is easy or convenient, and in fact most women who try to pump while they are at work end up abandoning the effort because it is complicated and so little support is available. In certain jobs (usually low-paying ones where workers have little autonomy) pumping may be impossible because there is no place to store milk safely and no place to pump with privacy. Most women who work outside the home and choose to breastfeed their infants would prefer that their babies were with them at work, at least when they are less than 1 year old. Ideally the profamily values touted in the United States in recent years would include pro-family work conditions that enable, rather than interfere with, breastfeeding. Lepore’s concern is that by promoting breast pumps and even allowing time for pumping at work, we are ‘‘avoiding harder—and divisive and more stubborn—social and economic issues’’ about parenting and the needs of women and their families.
As we have seen, breastfeeding is far more than a food delivery system, no matter how beneficial the food is for infant development. Psychologist Harry Harlow demonstrated decades ago that infants want soft, warm mothers in addition to milk. The sad little rhesus macaque babies he studied spent almost all of their time on the milk-less cloth ‘‘mothers,’’ leaving them only to quickly nurse from the wire ‘‘mothers.’’ But that having been said, and despite much research that suggests health throughout life is positively impacted by breastfeeding, public health campaigns that focus on women and blame them for not breastfeeding miss the point that decisions about infant care are often out of individual control. Annie Is a teenage girl who opts to return to school and leave her infant in the care of her mother who feeds her formula making a ‘‘bad’’ decision? She trades the good qualities of her milk for the chance to improve her social and economic options through education. Which choice is most likely to improve the long-term health of her child? In the United States, education probably trumps mother’s milk in this case. In a pathogen-infested impoverished home, staying home and breastfeeding may be the better choice for infant survival. How about a woman who is the sole breadwinner for her family? If she does not return to work soon after birth, she and her family may lose their home, resort to subsisting on foods of inferior quality and quantity, and be unable to afford needed health care. Quite a trade-off for even the best things that mother’s milk does for maternal and infant health. Then consider the severely depressed mother who may be able to provide milk but does so under duress and for whom breastfeeding worsens depression or increases stress. I have presented evidence and arguments based on evolution that, all other things being equal, breastfeeding is the very best option for feeding infants, but humans throughout evolution have rarely been able to pursue the optimal or best strategy. Far more common is the ‘‘good enough’’ strategy. Under circumstances where breastfeeding interferes with family health and well-being, formula feeding is certainly good enough, especially if the warmth and emotional interaction of breastfeeding is provided by the person holding the bottle. If breastfeeding is problematic because it interferes with golfing, partying, and horseback riding, maybe it would be advisable to reconsider the options.
Another technological fix that seems to derive from concerns about parenting is a machine that monitors an infant’s cry, interprets it, and informs the parents what the infant is trying to say with his cries. In other words, it translates baby communication into a ‘‘language’’ that parents can understand; it solves the problem that arises from a crying baby who ‘‘is no longer merely a being to be loved, but a problem to be solved.’’ For about $100, you can buy a calculator-sized device that will interpret cries and categorize them into five possible meanings: hungry, sleepy, uncomfortable, stressed, or bored. To work correctly, the device needs to be positioned about two yards from the baby (the device comes with a chart that tells you how far to place it depending on the baby’s weight) and it takes about 20 seconds to produce a translation. A light on the monitor tells you that the signal is being processed and changes colour when the diagnosis is complete. If you are not sure of the diagnosis, you can look at the baby’s body language, consult another chart, and come to a conclusion. Presumably, once the parents know why the baby is crying, they can respond appropriately (there is yet another chart that lists possible ways of calming the baby), although the delayed response may result in a different type of cry (a change from bored to stressed). Many agree that this gadget has nothing to do with communication, which is a two-way process between infant and parent and which is at the base of what it means to be human. For those who say that the device will reduce parental anxiety, I offer the suggestion that a few days spent with the baby, communicating with him in a two-way fashion, would probably be a better way to overcome most anxieties about what the baby wants when he cries. Another example of where technological innovation seems misguided is the one-way baby monitor that is placed in the infant’s room while he or she is sleeping, enabling parents to monitor the sounds the babies make while they cook, clean house, read, sleep (in their own rooms), or otherwise carry on their daily lives. Jim McKenna claims that based on our understandings of the evolutionary and cross-cultural environments of infancy, the monitor, if it is used at all, should be turned the other way. He argues that infant sleep, arousals, heart rate, and breathing all benefit from hearing the sounds that are part of normal living. Of course, the argument from an evolutionary perspectiv is that infants should be physically with their parents almost all of the time, in which case exposure to the sounds of everyday living is a given. But the reality of many of our lives is that we cannot keep our babies with us at all times, and we may not feel comfortable having the baby in our beds or even in our rooms at night. In this case, a monitor that broadcasts both ways may be the solution. Parents can hear the baby and respond if necessary and he can hear the sounds his parents and other family members make.
Perhaps not so egregious, but still a long way from ancestral infancy, are products aimed at increasing an infant’s intelligence by exposing him to music, ‘‘educational’’ videos, television, and other passive stimuli. Some of the advertising for these products claims that it will increase intelligence, but most of the research suggests otherwise. In fact, not only is there little evidence of positive effects on intelligence, but there is more evidence of potential harm to infant development from too much exposure to visual media. Early language development and other aspects of intelligence proceed well enough in the context of visual, aural, and other aspects of communication with a real person, especially parents. This was the context in which infant skills developed for several million years of evolutionary history. In evolutionary medicine, a question that is often asked is, ‘‘what is the evolutionary environment’’ of a given behaviour—meaning what were the ancestral conditions under which it may have evolved? For infants, the evolutionary environment was and is the mother’s body. Any measure of infant development that hopes to reveal normalcy must consider how it occurs when the infant is in contact with the mother, being held or carried by her, breastfed by her, or sleeping with her. Under these conditions, infants feed frequently, rarely cry, almost never die from SIDS, sleep on their backs or sides so as to facilitate nursing, awake frequently and fleetingly in the night, and grow up to be healthy children and adults. Because human infants are relatively helpless at birth and dependent on parental care for as much as a decade or more, a mother’s commitment of time and energy does not stop when she weans her child, although after that, other family and community members can provide comparable care. In fact, caretaking assistance from others was likely an important key to the mother’s ability to bear another child and to begin the intense direct-investment-in-infants cycle again. Evidence from studies of other primates and human foragers indicates that infants are in near-constant contact with their mothers or another person for the first three to four years of their lives. Consider that a mother who carries her infant while she forages must obtain enough calories to support her own metabolism and energy expenditure, including that required to carry her infant, and to produce sufficient milk. As the infant grows, he or she gets heavier and heavier until the time arrives for weaning, after which time others can assist in child care. Once the baby is weaned and the mother is freed from lactation and carrying an infant, if she continues to get approximately the same amount of calories, sufficient reserves can be built up so that ovulation occurs again, leading to pregnancy, breastfeeding, and carrying a new infant.