2 Evolution of Human Growth

Ms. Heemanshu Aurora

1. INTRODUCTION

Biological evolution is the continuous process of genetic and genomic adaptation of organisms to their environments. Natural selection determines the direction of evolutionary change and operates by differential mortality between individual organisms prior to reproductive maturation and by differential fertility of mature organisms (Cameron and Bogin, 2012).

Human growth, development, and maturation have evolved, sometimes as discrete processes, but more often as an integrated series of biological events. Dobzhansky (1973) said that, ―nothing in biology makes sense except in the light of evolution.‖

The study of the growth of human body holds utmost importance for evolutionary biologists because in multicellular organisms, major evolutionary change proceeds by alterations in the pattern of growth, development, and maturation. The human species is no exception (B. Bogin, 2010). The human pattern of growth and development appears to differ markedly from pattern of growth in other primates‘ species in the following ways:

1. Humans have four stages of growth and development between birth and adulthood. These are these are infancy, childhood, juvenile, and adolescent.
2. The infancy and juvenile stages are shared with most non-human primates, social carnivores, elephants, and many cetaceans. The childhood and adolescence stages are human species-specific features.
3. Human childhood and adolescence evolved because they confer reproductive advantages, increasing the fertility of the parents and reducing the mortality of their offspring. This is classic natural selection.
4. Adolescence may have evolved by both natural selection and sexual selection. Adolescent‘s may contribute significant amounts of food and labour to their families and this enhances reproduction by the parents and survival of their offspring (natural selection). The sex-specific features of adolescent girls and boys enhances opportunities for an apprenticeship-type of learning and practice of the wide variety of economic, social, political, and sexual skills needed for their own adulthood and successful reproduction (sexual selection).

2. LIFE HISTORY THEORY

It is a branch of biology that studies the selective forces that have guided the evolution of the schedule and duration of key events in an organism‘s lifetime as these relate to investment in growth, reproduction and survivorship. Each species has its own life history that is a pattern of allocation of energy towards growth, maintenance, reproduction, raising offspring to independence and avoiding death. It is important to emphasize that this pattern is not conscious or planned by the individual, group of individual or species. Rather it constitutes the set of biological and behavioural traits that characterize a species and which evolve via a series of tradeoffs, that is, costs to an organism‘s fitness in one trait due to invest in another trait (Kozlowskiand Weigert, 1986; Sterns 1989, 1992; CharnovandBerrigan, 1993; Roff, 1992; Walker, Gurven, Burgerand Hamilton, 2008).

Fig 1.Average Velocity and Distance Curves of Growth in Height for Healthy Girls (Dashed Lines) and boys (Solid Lines), Showing The Postnatal Stages Of Human Growth. Values For The Distance Curves Are On the Left Y-Axis; Values For The Velocity Curve Are On The Right Y-Axis. In The Velocity Curves, Note the spurts In Growth Rate At Mid-Childhood And Adolescence For Both Girls And Boys. The Postnatal Stages: I, Infancy; C, Childhood; J, Juvenile; a, Adolescence; M, Mature Adult. In Traditional Human societies, Weaning (W) Of Infants From Any Breastfeeding Occurs At An Average Age Of 30 Months, With Arrange Of 6–60 Or More Months (Dettwyler 1995; Sellen 2006). Figure Based On Bogin (1999).

The figure shown illustrates the amount ofgrowth, or distance, and rate of growth, or velocity, of healthy human beings from birth until adulthood. The velocity changes in growth correspond with stages of human life history.The infancy stage begins at birth and lasts until about 3.0 years of age. Postnatal infant growth is rapid, as is its rate of deceleration. Human childhood encompassesthe ages of about 3.0–7.0 years. Body growthduring childhood proceeds at a steady rate of 5–6 cmper year. Brain growth is rapid and the difference in growth rates is an example of a life history trade-offgiven limited energy. Many children experience a transientand small ―spurt‖ in growth rate as they transitioninto the juvenile period. Juvenile mammals are sexuallyimmature, but physically and mentally capable of providing for much of their own care. In many human societies, juveniles perform important work including food production and the care of children (i.e. ―babysitting‖).Juvenile growth rate declines until puberty, representing another trade-off between current growth ersus building a higher quality body and behavioural repertoire over a longer period of time. Puberty is a short-term event of the central nervous system, which initiates sexual maturation and the adolescent life stage (Bogin, 2010)

In humans, the hormones responsible for sexual maturation also cause the adolescent growth spurt in stature and other skeletal dimensions. The growth spurt, which is a notable feature of the adolescent growth stage, but not the only defining characteristic, begins at about 10.0years for girls and 12.0 for boys. The adolescent spurt and growth of the skeleton ends at about 18–19 years for girls and 20–22 years for boys and with this the adulthood, or reproductive stage of life history, begins. There are also significant changes in dentition, motor control, muscular development, cognitive function, and emotions associated with human infancy, childhood, juvenile and adolescent development. The integration of these separate domains of function also takes place with age and maturation (Bogin, 1999).

Anthropologists, human biologists, physicians and others have become increasingly interested in explaining the significance of Human life history. This interest is due to the discovery that several aspects of the human life cycle stand in sharp contrast to other species of social mammals, even other primates. Evolutionary theory needs to explain how humans successfully combined a vastly extended period of offspring dependency and delayed reproduction with helpless new-borns, a short duration of breastfeeding, an adolescent growth spurt and menopause. The evidences related to human evolution suggests that life history had evolved as a mosaic and may have taken form over the past two million years.(Cameron and Bogin,2012)

In humans research is mainly focused on postnatal stages of life cycle to study change in the rate of growth after birth and the timing of the onset of reproductive maturation. This is due to the fact majority of the mammal‘s progress from infancy to adulthood seamlessly without any intervening stages. Furthermore most mammals experience puberty after the peak velocity of postnatal growth. On the other hand human growth includes several stages between infancy and adulthood, and puberty occurs before the peak velocity of postnatal growth (Cameron and Bogin,2012).

STAGES OF HUMAN LIFE CYCLE

The stages of human life history between birth and adulthood are: Infancy, Childhood, Juvenile and adolescent. Each stage is related by characteristics of the dentition, changes related to method of feeding, physical  and  mental competencies, or maturation of the reproductive system and sexual behaviour

3.1 INFANCY

In infancy rate of growth is characterized by a steep deceleration in velocity. Motor skills i.e. sitting without support, crawling, walking with assistance develop rapidly during this stage. This phase of human life is also marked by eruption of deciduous dentition .Human infancy ends when the child is weaned from the breast, which in pre-industrialized societies occurs between 24 and 36 months of age. Complementary food ,which supplement mother‘s milk are introduced to the infant‘s diet sometime before the age of 1 year after birth, but nursing provides important nutrients ,immune substances and emotional support to the infant. By the end of infancy i.e. 3 years of age, the youngster can run short distances, pour water from a pitcher, and manipulate small objects. The development of the skeletal, musculature and nervous system accounts for all of these motor and cognitive advances. The rapid growth of the brain is most important during infancy because no other tissue or organ of the body grows more rapidly than brain as shown in figure 2.

Fig. 2. Growth curves for different body tissues. The ‗‗brain‘‘ curve is for total weight of the brain (Cabana et al., 1993).The ‗‗dentition‘‘ curve is the median maturity score for girls based on the seven left mandibular teeth (I1, I2, C, PM1, PM2, M1, M2) using the reference data of Demirjian (1978). The ‗‗body‘‘ curve represents growth in stature or total body weight. The ‗‗reproductive‘‘ curve represents the weight of the gonads and primary reproductive organs (Scammon, 1930).

3.2 CHILDHOOD

Childhood stage follows infancy and is demarcated by its own pattern of growth, feeding behaviour, motor skills and cognitive development. The slow progress of growth during infancy ends at the beginning of childhood and the growth rate levels off at around 5-6 cm per year. This slow and steady rate of human growth maintains a relatively small sized body during the childhood years. Human children require specially prepared food because of their immature dentition, the small size of their stomachs and intestines, and the rapid growth of their brain.( Cameron and bogin,2012) The human newborn uses 87% of its resting metabolic rate (RMR) for brain growth and function (Leonard and Roberson,1994). With age the usage of RMR for brain growth decreases, at the age of 5 years it is 44%  whereas in the adult human it is between 20 and 25%of RMR. Among chimpanzee at the similar stages of development the RMR values are about 45,20,and 9% respectively.(Leigh, 2004) Two of the important physical developmental milestone of human childhood is:

1. The replacement of the deciduous teeth with the eruption of the first permanent teeth.
2. Completion of brain growth.

In addition to these, cognitive and emotional capacities mature to new levels of sufficiency. Language and symbolic thinking skills mature rapidly. The mid-growth spurt is an another important feature of human childhood. This spurt is associated with an endocrine event called adrenarche, the progressive increase in the secretion of adrenal androgen hormones

.(Bogyman Campbell) Adrenal androgens produce the mid-growth spurt in height, a transient acceleration of bone maturation, and the appearance of auxiliary and pubic hair, and seem to regulate the development of body fatness and fat distribution (Katz et al., 1985; Parker,1991). The mechanism controlling adrenarche is not understood because no known hormones appear to cause it. Whatever its cause, a mid growth spurt has not been detected in chimpanzees or other primates and it may be unique to humans.(Cameron and bogin,2012)

3.3 JUVENILE

The evolution of the juvenile stage of primates is associated with both social complexity and dietary complexity, including foraging for fruits and seeds etc.(Walker, Berger, Wagner and Von Rueden, 2006). Study of juvenile primates and human juveniles in many cultures indicate that much social learning takes place during this stage, and a ‗learning hypothesis‖ has often been proposed to account for the evolution of the juvenile stage. It has also been argued that the primary reason for the evolution of juvenility appears to be a strategy to avoid death from competition with older individuals while living in a social group (Janson and Van Schaik, 1993).

Among humans following the childhood stage the individual enters the juvenile stage, which may be defined as the time from about 7–10 years old for girls and 7–12 years old for boys.

Important developments that allow children to progress to the juvenile stage of growth and development are the eruption of the first permanent molars and completion of growth of the brain (in weight). First molar eruption takes place, on average, between the ages of 5.5 and 6.5 years in most human populations (Jaiswal, 1983; Smith, 1992). Functional occlusion occurs some weeks to months thereafter. Recent morphological and mathematical investigation shows that brain growth in weight is complete at a mean age of 7 years (Cabana et al., 1993). Thus, significant milestones of dental and brain maturation take place at about 7 years of age.

At this stage of development juveniles have sufficient maturity of many body systems, such as the dental, locomotors, and cognitive systems, to allow for self-feeding capability. Much important learning of economic and social skills takes place during this stage. Juvenility ends with puberty, which is an event of short duration (days or a few weeks) that takes place in the brain, within or near the hypothalamic-pituitary axis, and results in a reactivation within the central nervous system of sexual development. This includes a dramatic increase in secretion of sex hormones, which is one marker of the onset of adolescence (Bogin, 2010).

The adolescent life history stage of growth, development, and maturation lasts 5–10 years after the onset of puberty: Other notable features of adolescence include a growth spurt in height and weight, the completion of permanent tooth eruption, development of secondary sexual characteristics (fat and muscle typical of each sex), and the intensification of interest in and practice of adult social, economic, and sexual activities leading to socio sexual maturation. Adolescence ends with the cessation of skeletal growth in length (the closing of the epiphyses of the long bones), the completion of dental development (eruption of the third molar, if it is present), and sexual maturation (measured for women as the age at first reproduction). On a worldwide basis, including living and historical societies, the age of onset of adulthood averages 19 years for women and21–25 years for men (Bogin, 2001).

3.4 ADOLESCENCE

Human adolescence begins with puberty, it is a stage of life cycle where social and sexual maturation takes place. This stage also includes development of secondary sexual characteristics such as development of the external genitalia, sexual dimorphism in body size and composition and the onset of greater interest and practice of adult patterns of socio-sexual and economic behaviour.

The adolescence phase in primates is different from human adolescence by two ways: Firstly the length of time between age at puberty and age at first birth. Humans take, on an average, at least 10 years for this transition. (Bogin, 1999; Bogin, 2001; Walker, Gurven, Hill, Migliano, Chagnon and DeSouza,2006).With reference to studies done worldwide , the average age for the first external manifestations of puberty for healthy girls is 9 years and first birth is at 19 years. Similarly, boys show external signs of puberty, on an average at 11 years and fatherhood at 21- 25 years. The point to make here is that monkeys and apes take less than 3 years to make the transition from puberty to parenthood.(Cameron and Bogin)

The second human difference is that during this life stage, both boys and girls experience a rapid acceleration in the growth velocity of skeletal tissues i.e. the adolescent growth spurt. The magnitude of this acceleration in growth was calculated for a sample of healthy Swiss boys and girls measured annually between the ages of 4 and 18 years. At the peak of their adolescent growth spurt, the average velocity of growth in height was 9.0cm/year (3.5 inches/ year) for boys and 7.1 cm/year(2.8 inches/year) for girls. (Largo, Gasser, Prodder, Stutzle, W Huber, 1978). Similar average values are found for adolescents in all human populations. No other primate species, not even chimpanzees, exhibit this pattern of skeletal growth.

3.5 ADULTHOOD

This stage begins with the completion of the growth spurt, the attainment of adult stature and fullreproductive maturity i.e. physical and psychosocial maturity.(Bogin,1988,1993, 1994a). Height growth stops when the long bones of the skeleton and the vertebral bodies of the backbone lose their ability to increase in length. Usually this occurs when the epiphysis, the growing end of the bone, fuses with the diaphysis, the shaft of the bone. The fusion of epiphysis and diaphysis is stimulated by gonadal hormones, the androgens and estrogens. These are also the hormones that promote reproductive maturation of the body and the brain.

The course of growth and development during the prime reproductive years of adulthood are relatively uneventful.(Cameron and Bogin,2012)

3.6 LATER LIFESTAGES

Old age and senescence follows the prime years of adulthood. The aging period is associated with gradual or sometimes rapid decline in the ability to adapt to environmental stress. Unlike the biological regulation of growth and development prior to adulthood , the aging process appears to follow no species specific uniform plan except menopause ,the event of later adult years that is experienced universally by women who live beyond 50 years of age ; men have no similar event.

3. ADDITION OF NEW STAGES IN HUMAN LIFE CYCLE DURING EVOLUTION

In size and cycle, Bonner develops the idea that the stages of the life cycle of individual organisms are ―the basic unit of natural selection‖. The presence of a stage and its duration in the life cycle relate to basic adaptations such as locomotion, reproductive rates and food acquisition. To make sense out of the pattern of human growth one must look for the ―basic adaptations‘‘ that Bonner describes. The most basic of these adaptations are those that relate to evolutionary success. This is traditionally measured in terms of the number of offspring that survive and reproduce. Biological and behavioural traits do not evolve unless they confer upon their owners some degree of reproductive advantage, in terms of survivors a generation or more later. (Bonner, 1965)

4.1 HOW DO CHILDHOOD STAGE EVOLVED

From the evidences on the life cycle of extinct species inferences for the hominids, based on comparative anatomy, comparative physiology, comparativeethology, and archaeology hypothesized patterns of brain and body growth in apes, humans, and their ancestors. Apes have a pattern of brain growth that is rapid before birth and relatively slower after birth. In contrast, humans have rapid brain growth both before and after birth. (as shown in Fig.3)

Fig.3. Growth curve for human brain and body compared to that of the chimpanzee. The length of the human foetal phase, in which brain and body grow at the same rate for both species, is extended for humans Chimpanzee brain growth slows after birth, but humans maintain the high rate of brain growth during the postnatal phase. In contrast, the rate of human body growth slows after birth. If the human brain/body growth rate were equal to the chimpanzee rate, then adult humans would weigh 454 kg and stand nearly 3.1m tall

The human-ape difference is not due to any single heterochronic process, that is, not the result of delay, prolongation, or acceleration of a basic ape-like pattern of growth. Rather it is due to new patterns of growth for the human species. Martin (1983) hypothesizes that a ‗‗humanlike ‗pattern of brain and body growth becomes necessary once adult hominid brain size reaches about 850 cc. This 850cc adult brain size maybe achieved by all hominoids, including extinct hominids, by lengthening the foetal stage of growth. At brain sizes above 850 cc the size of the pelvic inlet of the fossil hominids, and living people, does not allow for sufficient foetal growth. Thus, a period of rapid postnatal brain growth and slow body growth-the human pattern-is needed to reach adult brain size. Brain growth for both apes and humans end at the start of the juvenile stage, which means that apes complete brain growth during infancy. Humans, however, insert the childhood stage between the infant and juvenile stages. Childhood may provide the time and the continuation of parental investment, necessary to grow the larger human brain.

The patterns of growth of the fossil hominid species are reconstructions based on published analyses of skeletal and dental development of fossil specimens that died before reaching adulthood The prolongation of the total time for growth that plays such a prominent role however is not sufficient enough to account for the insertion of the new stages of childhood and adolescence that are part of human growth. (Smith, 1991b, 1992; Bogin and Smith, 1996).

Australopithecus Afarensis is considered by most palaeontologists to be a hominid, but shares many anatomical features with non-hominid pongid species including an adult brain size of about 400 cc (Simons, 1989) and a pattern of dental development in distinguishable from extant apes (Smith, 1991b).Therefore, the chimpanzee and A. Afarensis are depicted in Figure 12 as sharing the typical tripartite stages of postnatal growth of social mammals—infant, juvenile, adult(Pereira and Fairbanks, 1993). To achieve the larger adult brain size of A. africanus (442 cc) may have required an addition to the length of the foetal and/or infancy periods. The rapid expansion of adult brain size during the time of Homo habilis (650 to 800cc) might have been achieved with further expansion of both the foetal and infancy periods. However, the insertion of a brief childhood stage into hominid life history may have occurred. This conjecture is based on a comparison of human and ape reproductive strategies. There are limits to amount of delay between birth and sexual maturity, and between successful births, that any species can tolerate. The great apes are examples of this limit. Chimpanzee females in the wild reach menarche (the first menstruation) at 11 to12 years of age and have their first births at an average age of 14 years (Goodall, 1983).The average period between successful birth sin the wild is 5.6 years and young chimpanzees are dependent on their mothers for about 5 years (Teleki et al., 1976; Goodall,1983; Nishida et al., 1990). Actuarial data collected on wild-living animals indicate that between 35 percent (Goodall, 1983) and 38percent (Nishida et al., 1990) of all live-born chimpanzees survive to their mid-20s. Although this is a significantly greater percentage of survival than for most other species of animals, the chimpanzee is at a reproductive threshold. Goodall (1983) reports that for the period 1965 to 1980 there were 51births and 49 deaths in one community of wild chimpanzees at the Gombe Stream National Park, Tanzania. During a 10-yearperiod at the Mahale Mountains National Park, Tanzania Nishida et al. (1990) observed‗‗74 births, 74 deaths, 14 immigrations and 13 emigrations‘‘ in one community. Lovejoy (1981) calls the plight of great ape reproduction a ‗‗demographic dilemma‘. The great apes, and fossil hominids such as Australopithecus or early Homo, seem to have reached this demographic dilemma by extending infancy, forcing a demand on nursing to its limit (Fig. 4). Early Homo may have overcome this demographic dilemma by reducing the length of infancy and inserting childhood between the end of infancy and the juvenile period.

Fig.4. The Evolution Ofhominid Life History During Thefirst 20 Years Of Life. Abbreviated Nomenclature As Follows:A. Afar., Australopithecus Afarensis; A. Africa., Australopithecusafricanus; H. Habilis,Homo Habilis; H. Erec. 1, earlyHomo erectus; H. Erec.2, Latehomo Erectus; H. Sapiens,Homo-sapiens.

This certainly occurs among modern humans. An often cited example, the! Kung, are a traditional hunting and gathering society of southern Africa. A!Kung woman‘s age at her first birth averages 19 years and subsequent births follow about every 3.6 years, resulting in an average fertility rate of 4.7 children per woman (Howell, 1979; Short, 1976)

An increase in brain size from 850 to 900 cc with the evolution of homo erectus justify the insertion of the childhood period to provide the biological time needed for the rapid, human-like, pattern of brain growth. It should be noted from Figure.4 that the model of human evolution proposed here predicts that from the Australopithecus to the H. erectus stage the infancy period shrinks as the childhood stage expands.

Bogin (1988) lists following reasons for the evolution of human childhood from the perspective of reproductive success.

• 1) An extended period for brain growth; 2) time for the acquisition of technical skills, e.g. tool making and food processing and 3) time for socialization, play, and the development of complex social roles and cultural behavior. Insertion of childhood maybe better viewed as a feeding and reproductive adaptation for the parents of the child, as a strategy to elicit parental care after infancy, as a strategy to minimize the risks of starvation for the child, a means of shift in the care of offspring from the parents, especially the mother, to juveniles and older post reproductive, adults (i.e., grandmothers),and as a mechanism that allows for more precise ‗‗tracking‘‘ of ecological conditions via developmental plasticity during the growing years. Thus in addition to the above mentioned three ‗explanations, there are at least five additional reasons for the evolution of childhood
• 4) Childhood is a feeding and reproductive adaptation: The evolution of childhood as a stage in human life history ‗‗fills the gap‘‘ between the infant‘s dependency on the mother for food via nursing and the feeding independence of the juvenile. This frees the mother from the demands of nursing and the inhibition of ovulation related to continuous nursing. This decreases the inter birth interval and increases reproductive fitness.
• 5) A stimulus to release these parental behaviors toward children may be found in the very pattern of growth of the children themselves, in that the allometry of the growth of the human child releases nurturing and care-giving behaviors in older individuals. The human child maintains an infantile appearance (large cranium, small face and body, little sexual development), which stimulates nurturing and care-giving behaviors in older individuals. A series of ethological observations (Lorenz, 1971) and psychological experiments (Todd et al., 1980; Alley, 1983) demonstrate that these growth patterns of body, face, and brain allow the human child to maintain a superficially infantile (i.e., ‗‗cute‘‘) appearance longer than any other mammalian species children (Bogin, 1988, 1990; McCabe, 1988).
• 6) Children are relatively inexpensive to feed. The relatively slow rate of body growth and small bodysize of children reduces competition with adults for food resources, because slow-growing, small children require less total food than bigger individuals. A 5-year-old child of average size (the 50th centile of the NCHS reference curves for growth) and activity, for example, requires 22.7 percent less dietary energy per day for maintenance and growth than a 10-year-old juvenile on the 50th growth centile (Ulijaszekand Strickland, 1993; Guthrie and Picciano, 1995). Thus, provisioning children, though time consuming, is not as laborious a task of investment as it would be, for instance, if both brain and body growth were progressing at the same rapid rate. 
• 7) The task of child care becomes even less tiring because ‗‗babysitting‘‘ is possible. Since children do not require nursing any competent member of a social group can provide food and care for them. Early neurological maturity vs. late sexual maturity allows juveniles and young adolescents to provide much of their own care and also provide care for children (Bogin, 1994a).Grandmothers and other post reproductive women also provide much child care (Bogin and Smith, 1996). Again, this frees younger adults, especially the mother, for subsistence activity, adult social behaviors, and further childbearing.4)
• 8) A further important reason for the evolution of childhood is that childhood allows for developmental plasticity. The term plasticity means a potential for change in the phenotype of the individual caused by a change in the environment.(Stearns 1992;Mascie-Taylor and Bogin 1995). The fitness of a given phenotype varies across the range of variation of an environment. When phenotypes are fixed early in development, such as in mammals that mature sexually soon after weaning (e.g., rodents), environmental change and high mortality are positively correlated. Social mammals (carnivores, elephants, primates) prolong the developmental period by adding a juvenile stage between infancy and adulthood. Social mammals (carnivores, elephants, primates) prolong the developmental period by adding a juvenile stage between infancy and adulthood. They experience a wider range of environmental variation, and the result is a better conformation between the individual and the environment. Fitness is increased in that more offspring survive to reproductive age than in mammalian species without a juvenile stage (Lancaster and Lancaster, 1983; Pereira and Fairbanks, 1993).Humans insert the childhood stage between infancy and the juvenile period. This results in an additional 4 years of relatively slow physical growth and allows for behavioral experience that further enhances developmental plasticity. The combined result is increased fitness (reproductive success). (Lancaster and Lancaster, 1983)

4.2 EVOLUTION OF ADOLESCENCE STAGE

An adolescent stage of human growth may have evolved by the process of natural selection and sexual selection. Both types of selection were identified by Charles Darwin. Where Natural selection operates to increase the frequency of genotypes and phenotypes that confer reproductive advantages on the individuals possessing them, sexual selection depends on the advantage which certain individuals have over individuals of the same sex and species , in exclusive relation to reproduction.(Darwin,1872).

Adolescence may be a key in helping the next generation rear its young successfully. This stage may provide Boys and Girls with a life history strategy to survive to adulthood, the immediate benefit and as a long term benefit to practice complex social skills required for effective mating and parenting. Sexual selection may have acted on the ―mating‖, and natural selection on the ―parenting‖. There are certain trade- offs associated with adolescent stage, such as the delay in the onset of reproduction and risk for damage and death. It seems these risks may have overweighed by the reproductive advantages of adding adolescence to human life history.

4.2.1 ADOLESCENCE AS APPRENTICESHIP

Some life history theorists hypothesize that an the adolescent stage of human growth evolved to provide the time to learn and practice complex economic, social, and sexual skills required for effective food production and reproduction and parenting (Bogin,1993, 1999). In this perspective, adolescence is a time for an apprenticeship, working and learning alongside older and more experienced members of the social group. The benefits of the skills acquired during adolescence are lower mortality of both first-time mothers and their offspring. This places the ―apprenticeship hypothesis‖ for the learning and practice value of adolescence firmly within Darwinian natural selection theory and it is likely that the learning and practice of adult skills play an important role in human growth, development and maturation.

4.2.2 UNUSUAL GROWTH OF THE HUMAN PELVIS

A fundamental of human growth that delays female fertility until the late teenage years is the growth of the pelvis. Ellison (1982) and Worth man (1993) found that age at menarche is best predicted by bi-iliac width, the distance between the iliac crests of the pelvis. A median width of 24 cm is needed for menarche in American girls living in Berkeley, California, Kikuyu girls of East Africa, and Bundi girls of highland New Guinea. The pelvic width constant occurs at different ages in these three cultures, about 13, 16, and 17 years old, respectively. This unusual pattern of pelvic growth can be understood in the light of bipedal walking, another special human attribute. Bipedalism is known to have changed the shape of the human pelvis from the basic ape-like shape. Apes have a cylindrical-shaped pelvis, but humans have a bowl shaped pelvis. The human shape is more efficient for bipedal locomotion but less efficient for reproduction because it restricts the size of the birth canal. Human women may need a longer period of pelvic growth to compensate for the restriction. The cross-cultural studies of reproductive behavior shows that human societies acknowledge (consciously or not) this special pattern of pelvic growth.

4.2.3 TWO PATHWAYS THROUGH ADOLESCENCE

Natural and sexual selection for adolescence applies to both girls and boys. The forces of natural selection are the same for both sexes but the particulars of sexual selection are different. The adolescent development of boys is quite distinct from that of girls. Girls look reproductively mature years before they are actually fertile. In contrast boys become fertile well before they assume the size and the physical characteristics of men. Analysis of urine samples from boys 11–16 years old show that they begin producing sperm at a median age of 13.4 years (Muller et al., 1989).The delay between sperm production and reproductive maturity is not wasted time in either a biological or social sense. The obvious and the subtle psycho physiological effects of testosterone and other androgen hormones that are released after gonadal maturation may ―prime‖ boys to be receptive to their future roles as men.

4.3 EVOLUTION OF MENOPAUSE

In addition to childhood and adolescence, human life history has another unusual aspect i.e. menopause combined with the potential of many years of healthy post reproductive life. In contrast to the non – human female primates, the human reproductive system is shut down well before the other systems of the body. Human women may be healthy at menopause and may have 20 or more years of relatively active life following menopause. (Cameron and Bogin, 2012).The inevitabilities of primate biology, combined with the creativity of human culture allow women of our species to develop bio cultural strategies to take greatest advantage of a post reproductive life stage.

5. SUMMARY

This chapter takes a life history approach to the study of human growth and development. The postnatal life cycle of social mammals including the non-human primates has three basic stages of postnatal development: infant, juvenile, and adult. The human life cycle, however is best described by six stages: infant, child, juvenile adolescent, adult and post reproductive women. The new life stages of human life cycle represent feeding and reproductive specialization. Hominins prior to genus Homo probably had life histories similar to living apes than humans Part of the reproductive success of the human species is due to the intense investment and care those parents, and other individuals, lavish on infants and children. In the course of human evolution, at least since the appearance of the genus Homo in the last 2 million years, patterns of growth were shaped by natural selection to promote and enhance Parental investment. One way this was accomplished was by stimulating what may be called the ‗‗psychology of parenting. Moreover, the complete ―package‖ of modern life history, with our distinctive bio cultural reproduction and long post reproductive life span for women took shape with the evolution of modern Homo sapiens around 125,000 BP.