5 CHILDHOOD

The childhood stage is peculiar to humans and has been defi ned by the stabilization of the growth rate, immature dentition, and weaning (while continuing to depend on older people for food), and on behavioral characteristics, including immature motor control (Bogin 1999 a). The human childhood stage is among the cornerstones of humans ’ enormous evolutionary fi tness. As a consequence of the childhood stage, more humans survive to adulthood than any other mammal. In addition to bipedal-ism, large brains and heads, and a spoken language, I view childhood among the foundations that defi ne us as humans.

This period between infancy and juvenility is marked by sex hormones quies-cence. The levels of testosterone in the boy and estrogen in the girl are extremely low, as is the concentration of the child ’ s gonadotropins. During this stage, the hypothalamic – pituitary – gonadal axis is extremely sensitive to negative feedback by estrogen, thereby suppressing sex steroid levels to their lowest levels. In this quies-cence of reproductive hormones, growth is faster and more stable as compared to that during the following juvenility period, when adrenal androgens appear and adiposity rebounds, consuming energy.

A. THE WEANLING ’ S DILEMMA

The vulnerability produced by humans ’ premature birth to fulfi ll the need to defer skull growth to the postnatal period has produced the “ the weanling ’ s dilemma ” (Kennedy 2005 ): Homo sapiens have a longer period of dependency than other hominids, yet are weaned from breast - feeding earlier than any ape. The life stage

Evo-Devo of Child Growth: Treatise on Child Growth and Human Evolution, First Edition. Ze’ev Hochberg.© 2012 Wiley-Blackwell. Published 2012 by John Wiley & Sons, Inc.

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of childhood emerges when the baby weans from breast - feeding, which for humans takes place at about 30 – 36 months of age, according to ethnographic observations in traditional societies and historical accounts. If we consider the extended life span of humans, the stage of infancy for Homo sapiens is markedly shorter than, for example, the 60 - month infancy of the chimpanzee, which lives in the wild for about 25 years to have a mean adult life span after sexual maturity at age 10 of about 15 years (Hill, Boesch et al. 2001 ) (Fig. 1.D.2 ).

The evolutionary trade - off for the “ weanling ’ s dilemma ” has been the introduc-tion of an additional life history stage — childhood. Early weaning created an obvious hazard for the child, and even more so ever since hominids adopted carcasses as a new and valuable food approximately 2.6 million years ago, getting us into a danger-ous competition with other carnivores. Yet many investigators suppose that the increased acquisition of energy - rich meat is among the most important evolutionary events that initiated the origin and success of the genus Homo . The trade - off for the new hazard was intellectual development, which became the primary focus of hominids ’ evolutionary selection.

The great lapse took place some 500,000 years ago with Homo erectus , whose transition from infancy to childhood took place before the fi rst permanent molar tooth erupted. The new childhood stage was “ deducted ” from infancy; it reduced its length, and because of this gainful step, H. erectus enjoyed greater evolutionary fi tness than any previous hominid. His populations increased in size and began to spread throughout Africa and other regions of the Old World.

The evolutionary gain of childhood lay in the mother ’ s freedom to cease breast - feeding her 3 - year - old infant in order to become pregnant again. This enhancement of reproductive output required the social interaction of an extended family and tribal ties, and as is evident from the consequences of this evolutionary leap, it did not put the mother, her infant, or her older children at risk. The Igbo and Yoruba tribes of Nigeria say that “ it takes a village to raise a child. ”

Once the infant is weaned, the child is no longer solely dependent on his mother for provision. The entire family, and sometimes the entire tribe, share responsibility of provision and protection. Puppies and little kids look cute to most adults in all cultures, and cuteness elicits loving care by adults. Evolution has made these positive emotions a crucial element in the childhood life - history stage. In a study on the role of the extended family in a rural area of the Gambia, it was demonstrated that having a living mother, maternal grandmother, or elder sisters had a signifi cant positive effect on the survival probabilities of children, whereas having a living father, paternal grandmother, grandfathers, or elder brothers had no such effect (Sear, Steele et al. 2002 ). Family and tribal care is a unique hominid adaptation, for no other primate or mammal (elephants may be one such exception) is so actively involved in feeding the broader family ’ s young (Lancaster and Lancaster 1983 ). A father role is a much later development in human sociology. It was not until the post - Ice Age extinction of many wild animals and the transition to shepherds fami-lies that the female primary groups accepted men as being “ the father ” to provide maximum protection for his wife and children.

The long period of food provisioning and protection, extending from 3 to 6 years, largely defi nes the childhood stage of human life history. By the age of 7, the four permanent molars have usually erupted and the permanent incisors have begun to replace “ milk ” incisors (Fig. 4.B.4 ).

THE GRANDMOTHER THEORY 93

Toward the end of this stage, children become able to eat the same foods as adults, new learning and behavioral capabilities enable greater social independence, the gait becomes adult - like (Bogin 1999 a), and cognitive and emotional developments permit new levels of self - suffi ciency. Seven - year - olds can perform many basic tasks, including food preparation, infant care, and other domestic tasks with little or no supervision.

Jean Piaget defi ned childhood as the cognitive preoperational stage (Piaget 1936 ) (Table 4.1 ). The child increasingly uses verbal representation, but speech is egocen-tric. Applying his new knowledge of language, the child begins to use symbols to represent objects. Early in this stage he also personifi es objects. He is now better able to think about things and events that are not immediately present. He is ori-ented to the present, but still has diffi culty conceptualizing time. His thinking is infl uenced by fantasy — the way he ’ d like things to be — and he assumes that others see situations from his viewpoint. He takes in information and then changes it in his mind to fi t his ideas.

In summary, the evolutionary advantages of childhood are apparent and contrib-uted to the reproductive fi tness of those hominids who acquired it: (1) By reducing the length of infancy and adding childhood, hominid mothers gained greater lifetime fertility than any ape, and (2) as a consequence of the family - oriented childhood stage, more hominids who acquired childhood survived to adulthood than any other mammal. These fi tness arguments increased as childhood has prolonged from 1 – 2 years in the Homo habilis to 3 – 4 years in Homo sapiens sapiens.

B. THE GRANDMOTHER THEORY

Observing nature in the wild gives the impression that animals are mostly selfi shly hostile to each other. The truth is that cooperation or cooperative behaviors are common, and turn out to be benefi cial to other members of the same species. We have all observed the cooperation of ants and bees in their nests, but these are small and distant creatures. Tribal life is an important case in point of a cooperation model. The theory of “ kin selection ” refers to apparent strategies in evolution that favor the reproductive success of an organism ’ s relatives, even at a cost to their own sur-vival and/or reproduction (Hamilton 1963 ; Smith 1964 ). Genes for such cooperative operations are conserved because they help preserve their owners from extinction. The continuity of his genes is the only reward the altruistic individual receives.

That grandmothers behave altruistically is a cross - cultural social tenet. Otherwise, why does a human ’ s life history include two decades beyond reproduction? But women as young as their mid - 30s can be grandmothers, and we need to be reminded that it is very common that women are both mothers of their younger children and grandmothers relative to the offspring of their oldest children at the same time. In order to contribute to the well - being of both generations of children, this woman needs to be in residential contact with the children and able to perform work, espe-cially food collection but also the tasks of the village such as water collection, hut construction, childcare, food processing and cooking, and so on to meet the needs of her own and her adult child ’ s household. Despite some objections, it is now accepted that a postmenopausal life stage is evolution - based, which is suggested by the fact that the decline of ancestral age - specifi c fertility persisted in our genus,

94 CHILDHOOD

while senescence in other aspects of physiological performance slowed down (Hawkes 2003 ).

The grandmother theory is an attempt to explain why menopause, rare in mammal species, arose in human evolution and how a long post - fertile period up to one - third of a female ’ s life span could confer an evolutionary advantage (Williams 1957 ). The theory maintains that this unusual feature of human demography has evolved because of the contributions that the postmenopausal female makes toward the fi tness of her children and grandchildren. She can monitor the health and well - being of her child and grandchildren, and contribute to their well - being, and she turns out to be the best gatherer in the community. On a mere genetic basis, women retain greater evolutionary advantage by caring for their daughters, who carry 50% gene identity, and grandchildren (25% identity) than by investing in more of their own children or the risks of multipara birth. A study of the Hadza tribe, a small group of hunter - gatherers in Tanzania, showed that while caring for their daughters and grandchildren, women in their 50s – 70s and beyond are also among the most diligent members of the group, gathering more food than almost any of their tribal peers (Blurton Jones, Hawkes et al. 2002 ). Proponents of the grandmother theory main-tain that postmenopausal survival, like big brains and upright posture, is among the biological traits essential for classifying us as humans.

While the grandfather carries over the same 25% of his genes to his grandchil-dren, we have to distinguish between grandmothers and grandfathers. When women of the Ju! ’ hoansi San of the Kalahari Desert become widows, they are likely to concentrate all their efforts on assisting their adult children (and continuing to raise their own if any are still dependent), while men who are widowed are more likely to marry again and may be caught up in providing assistance to the new wife ’ s adult children. No doubt these matters differ according to the cultural rules of the par-ticular group, but they are likely to be important events in the lives of individuals from any hunter - gatherer group (Howell 2010 ).

It was claimed by others that most of the benefi ts to longevity of the grandmother derive from helping their offspring rather than their grandchildren — the so - called mother hypothesis. (Lancaster and Lancaster 1983 ). It was argued that rather than the grandmother being a determinant of our longevity, the reverse may be so — our longevity is a central determinant of the grandmother ’ s existence: The length of a female ’ s post - reproductive life span was refl ected in the reproductive success of her offspring and the survival of her grandchildren (Lahdenpera, Lummaa et al. 2004 ).

C. GROWTH OF THE CHILD

The end of the rapid growth deceleration of infancy marks the beginning of child-hood, when, quite uniquely to modern humans, the growth rate levels off to a quasilinear rate of an average of 7 cm per year with minimal if any acceleration or deceleration (Fig. 2.B.1 ). Sexual dimorphism is almost nonexistent in this stage of quiescent hypothalamic – pituitary – gonadal axis in the boy, and semi - quiescent in the girl (Klein, Baron et al. 1994 ). Despite 10 - fold higher estrogen levels, girls grow as fast as boys during childhood, with somewhat of an acceleration at the end of this phase [the so - called mid - childhood or mid - growth spurt (Molinari, Largo et al. 1980 )], before the ensuing juvenility slows down growth.

ENDOCRINE ASPECTS OF CHILDHOOD GROWTH 95

The stable growth of children defers the total energy costs of rearing them by the family and tribe, and appears to enable parents to support multiple dependents of different ages at any one time (Dean 2007 ). Were our growth curve more like that of chimpanzees, which transit directly from infancy to juvenility and do not show a stable period of growth, then it would cost various modern hunter - gatherer mothers between 1,267 and 1,503 kcal/day more to support the same number of offspring (Gavan 1953 ).

Preliminary data suggest that the duration of this stable growth period may have a major impact on fi nal height. Assuming equal growth and timely transition from infancy to childhood as well as during juvenility adolescence, each year of a child-hood period adds to the child ’ s fi nal height as much as 6.2 cm in boys and 6.9 cm in girls. A shorter childhood and earlier transition to juvenility in girls give boys a small advantage in total childhood growth. The story of the Pygmy will be told later, but it can be mentioned here that the Pygmy have a shorter childhood stage, and they transition earlier into juvenility and adolescence with signifi cant compromise of their adult height.

D. ENDOCRINE ASPECTS OF CHILDHOOD GROWTH

The driving force behind the transition from infancy to childhood growth in both boys and girls is the growth hormone – insulin - like growth factor - I axis. The infancy – childhood growth transition at a mean age of 9 months (also the age at which growth hormone becomes operational) refl ects the control of growth by the growth hormone – insulin - like growth factor - I axis endocrine axis and their target cell responsiveness (Karlberg and Albertsson - Wikland 1988 ; Wit and van Unen 1992 ; Hochberg 2002a ). The infancy – childhood growth transition occurs in parallel with a rise in serum levels of the growth hormone - dependent insulin - like growth factor - I and insulin - like growth factor - I binding protein - 3 during the second half of the fi rst year of life. Children with a delayed infancy – childhood growth transition show a delay in the 6 - to 12 - month rise of insulin - like growth factor - I levels (Wang and Chard 1992 ; Leger, Oury et al. 1996 ) (Fig. 4.E.7 ). Moreover, the growth transition is absent or very delayed in children with growth hormone defi ciency who receive no hormonal therapy (Karlberg and Albertsson - Wikland 1988 ).