1. Introduction
Population growth after the development of agriculture has been one of the most dramatic features of human evolution and the estimated population was 8,000,000 individuals (10,000 years ago) who inhabited the earth. By A.D. 1800, the world’s population reached 1,000,000,000. The traditional view (Childe, 1951) holds that the post Neolithic population growth resulted from improved health which may be attributed to the effect of agricultural food surpluses. Explanation of post Neolithic population growth has focused on changes in demographic parameters. Most researchers agree that the population growth during the Paleolithic was very slow. Despite difficulty in estimating Paleolithic and Neolithic population size, there have been a number of reasonable reconstructions. Lack of Paleolithic population growth has been explained by arguing that populations were experiencing maximum fertility and very high mortality. Neolithic population explosion resulted from improved nutrition and health; these acted to reduce mortality, and the change in demographic pattern led to a rapid increase in population (Armelago et al., 1991). Sex and age determination through skeletal Analysis have been a very important source of determinants of demographic variables for the past population growths.
2. Skeletal Analysis: Male and female adult skeletal morphology varies across a continuum. Sex and age can be determined from skeleton, which can be later helpful in assessing the number of Individuals, age at death, fertility and mortality ratios and population growth patterns.
2.1: Sex Determination from skull and Pelvis: Much of the morphological difference between the sexes is genetically determined and is the result of two separate processes of natural selection. The sexdetermining genes control the development of the primary sex organs which equip the individual for either the male or the female reproductive role. Because successful reproduction is the key to the survival of an individual’s genes, these sex differences are under close control by stabilizing selection. But the separate process of sexual selection also has an important influence on the visible or phenotypic differences between the sexes. The bony pelvis, including the hip bones and the sacrum, is the most reliable part of the adult skeleton for sex estimation because it is the only skeletal region which exhibits specific morphological adaptations to the different sexes’ reproductive capacities. In the adult-female pelvis the postural and locomotor functions of the pelvis are modified by the requirement for the birth canal to allow the passage of a fully developed neonate. This is achieved by additional transverse growth of the pelvic bones which occurs around the time of puberty. This differential growth is manifest both in the overall shape of the pelvis, and in a series of specific morphological traits, each of which contributes to the overall adaptation. After the pelvis, the skull (cranium and mandible) provides the next most reliable group of sex-specific morphological indicators. The principal sex-discriminating features of the human skull are related to overgrowth or hypertrophy of bone in males, and to the tendency for males to show greater development of musculature and articulations with more prominent muscle markings and larger joint surfaces. For most of the sexdiagnostic traits of the cranium, the presence of the trait indicates a male and its absence indicates a female. Skeletal hypertrophy is seen most clearly in the skull of the adult male in the supraorbital region, the nuchal region, the mastoid process, and the attachment areas for the muscle of mastication and in the development of the mental trigone (chin) on the mandible. When we move away From the pelvis and the skull, sex discrimination relies principally on the average size differences between the sexes, particularly in the sizes of the articular surfaces at the ends of the long bones. On average the muscle bulk and body weight of males is considerably greater than that of females, and muscle forces and static weight must be transmitted through the joint surfaces which as a consequence are on average larger in males than in females (Chamberlain, 2006).
2.2: Age estimation from Ossification of Bones: The developing human passes through a succession of well-defined stages. These are the embryonic stage, from conception to 8 weeks; the fetal stage, from 9 to 40 weeks gestational age; the neonatal stage, from birth to 1 month; infancy, from 2 to 11 months; early childhood, from1 to 5 years, later childhood from 6 to 11 years, adolescence from 12 to 17 years, and adulthood from 18 years onwards. Bones and teeth begin forming in the fetus well before birth. Ossification begins at eight weeks in utero (in the womb), and the deciduous teeth begin calcifying at fifteen weeks. Most of the bones of the skeleton are initially formed as a cartilage model, in the middle of which appears the primary centre of ossification. Secondary centres of ossification appear at the ends of the bone, and are separated from the primary centre by intervening growth plates. These secondary centres subsequently become the bony epiphyses that support the articular surfaces. Bone growth continues by the addition of new bone on surfaces facing the direction of bone growth, while resorption takes place on surfaces facing away from the growth direction. The crowns of the deciduous and the permanent teeth develop within the jaw, and the teeth subsequently develop roots as they erupt through the oral tissues into the mouth. Towards the end of puberty most of the bones stop growing and the cartilagenous growth plates underneath the epiphyses are converted to bone. The epiphyses fuse to their respective diaphysis at different times but mainly from about 12 years onwards. Some growth centres in the skull and vertebral column fuse much earlier, while the medial end of the clavicle, the epiphyses of the vertebral bodies and the growth plates between the segments of the body of the sacrum do not fuse until the mid to late 20s.Most epiphyses fuse between 1 and 2 years earlier in girls than in boys, and fusion itself is a protracted process rather than a sudden event (Chamberlain, 2006).
2.3: Age through fetuses and Dentition: During fetal and early neonatal life the bones grow quickly, with about 20mm added to the length of long bone diaphysis in just 10 weeks, compared to an average of 20mmper year during the postnatal growth period. Thus in principle the age at death of a fetus can be estimated accurately from measurements of the size of its bones. The deciduous teeth commence calcification between 13 and 20 weeks gestational age, but the crowns of these teeth do not complete formation until the postnatal period. Deciduous dental development proceeds faster in prenatal males than in females. The tooth crowns are formed within the jaw and then erupt through the gum into the mouth while the tooth roots grow downwards into the jaw. Stages of tooth crown and root development can be recognized either by visual inspection or from a radiograph of the jaw with the developing teeth inside it. Up to 14 discrete stages of tooth development can be recognized radiographically from the initial appearance of calcification of the tooth cusps through to closure of the root apex following eruption alternatively the height of the crown of the developing tooth can be measured and compared to standard growth curves calculated from known-age samples. The resorption of the roots of the deciduous teeth prior to their exfoliation can also be used as a chronological marker in children aged between about 5 and 12 years. As tooth development appears to be less influenced by dietary and other disturbances to growth than is the case for other tissues of the body, age estimates based on dental growth are considered more accurate and reliable than those based on bone growth. An additional source of data for age estimation in children can be found in dental microstructure. Teeth carry a record of their own development in the microscopic incremental structures in enamel, dentine and cementum. These structures are visible in microscopic thin sections and electron microscopic images of developing tooth surfaces. The incremental markers include the Striae of Retzius – weekly growth increments in the enamel which are expressed on the outer surfaces of the tooth as perikymata –and enamel prism cross-striations, which represent daily variations in enamel secretion rates. By counting the number of perikymata on the surface of a partially formed tooth, and adding about 26 weeks to account for the hidden increments that are buried in the occlusal enamel, plus a correction for the age at which enamel calcification begins, the age at death of a juvenile skeleton can be calculated (Chamberlain, 2006).
age of fetus through skull image determination of age of skull
3. Determination of Demographic Variables
Data from ancient populations, Europe and Asia, show that only a small proportion of the population lived beyond 50, most died at about sixty or seventy years old. In the case of the Middle East (Southern Levant), Hershkovitz and Gopher (2008), studied hundreds of skeletons that came from a hunter gatherer series of Natufian period (15,000 to 12,000 a. P.) and preceramic sedentary groups from the Neolithic (from 12,000 to 8350 a. P). For the former the estimated life expectancy at birth, assuming they are stationary population, is 24.6 years, while for the latter it is 25.4 years. The average age at death of adult individuals represented in the series is 32.1 years to the hunter-gatherers (37.6 years for men and 30.1 years for women) and 31.2 years for the pre-ceramic group (men 32.2 and 35.5 for women). In Roman Egypt, found that only about one fifth of women lived from adolescence to age 60. The average life expectancy at birth was 25.60 and 22.5 for men and women respectively. At 30 years old this indicator is 22.23 and 24.998 respectively, therefore the probability of survival beyond the age of fifty was very low. From the study of Chinese genealogies, showed that very few people reached the age of 95 years and life expectancy at ages 30 and 50, were lower (Hernandez Espinoza et al, 2013).
3.1: Mortality: It have been noted that age at death distributions constructed from skeletal remains typically look significantly different than those produced by historical demographers, even though the historical and archaeological samples may not be that far removed in time. Paleodemographers, however, have typically reported unexpectedly low infant mortality and adult age specific mortality that is abnormally high and that accelerates rapidly from age 15 to 50, resulting in few individuals surviving into old age. Some anthropologists assert that mortality patterns in archaeological populations were significantly different than those observable in historic and modern anthropological populations. If skeletal elements are missing or incomplete, transition analysis allows for age estimates using only the features that are observable. One of the most significant benefits is the promise of improved estimates of age for older adults. Many methods of adult age estimation have an open-ended terminal age category. While these open-ended categories acknowledge the difficulties of accurately aging older individuals, they also limit the information regarding senescent mortality that can be obtained from the skeletal sample (Bullock et al., 2013).
Evidence suggests that the process of development since the Neolithic Revolution was indeed accompanied by an increase in the extrinsic mortality risk. For instance, skeletal remains from the preColombian America, analysed by Steckel (2004), demonstrate a decline in the health environment over the time period 6,000 BCE until 1500 CE. Importantly, differences in the timing of the Neolithic Revolution across regions generated significant variations in the genetic composition of the contemporary human population. For example, lactose tolerance was developed among European and Near Easterners since the domestication of dairy animals in the course of the Agricultural Revolution, whereas in regions that were exposed to dairy animals in later stages a larger proportion of the adult population suffers from lactose intolerance. The probability that an individual would survive to a reproduction age is affected positively by the genetically pre-determined somatic investment, and negatively by the extrinsic mortality risk that is associated with socio-environmental characteristics, which were altered by the Neolithic Revolution. A rise in mortality risk triggers a process of natural selection that alters the distribution of types within the population. Nature selects the level of somatic investment and thereby life expectancy that maximizes reproduction success in any given environment, and the distribution of these hereditary life-history traits evolves over time due to changes in the environment. As long as the adverse effect of population density on the survival probability is lower for individuals who are genetically pre-disposed for higher somatic investment, the evolutionary optimal level of somatic investment is an increasing function of the extrinsic mortality risk. Thus, the rise in the extrinsic mortality risk in the course of the Neolithic Revolution shifted the evolutionary advantage towards individuals with higher somatic investment and thus higher life expectancy. Furthermore, the rise in the extrinsic mortality risk and its interaction with the forces of natural selection induced a non-monotonic time path of life expectancy. During the Neolithic Revolution life expectancy declined, as long as the extrinsic mortality risked increased and the distribution of types in the population did not evolve considerably. However, the onset of the evolutionary process increased the prevalence of individuals with higher somatic investment in the population and ultimately generated a rise in life expectancy. The transition from the Palaeolithic period to the Mesolithic huntergatherer economies corresponded to the disappearance of large game animals and the consequent adoption of broad spectrum foraging patterns aimed at a wider array of small animals, seeds, and aquatic foods resulting in a decline in nutrition and thus human health and thus lead to mortality (Galor & Moav, 2007).
3.2: Life Expectancy: The rise in population density, the domestication of animals, and the increase in work effort in the course of the Neolithic Revolution increased the exposure and the vulnerability of humans to environmental hazards, such as infectious diseases, and led to the decline in life expectancy during that period. However, that in light of the fundamental trade-off between current and future reproduction, the Neolithic transition altered the evolutionary optimal allocation of resources towards somatic investment, repairs, and maintenance (e.g., enhanced immune system, DNA repairs, accurate gene regulation, tumor suppression, and antioxidants). The rise in the extrinsic mortality risk (i.e., risk associated with external environmental factors, as opposed to internal biochemical decay) in the course of the Neolithic Revolution generated an evolutionary advantage to individuals who were genetically pre-disposed towards higher somatic investment, increasing their representation in the population, and leading to the observed increase in life expectancy in the post-Neolithic period. The presumption predicts that the interaction between the rise in the extrinsic mortality risk and the evolutionary process manifests itself in the observed non-monotonic time path of life expectancy. In the short run — while the composition of the population remains nearly stationary: a rise in the mortality risk reduces life expectancy. However, the evolutionary process that is triggered by the environmental change generates an evolutionary advantage for individuals characterized by higher life expectancy, increasing their representation in the population. As the composition of the population shifts sufficiently in favour of individuals with higher life expectancy, the population’s life expectancy increases and ultimately it could reach a higher level than the one existed prior to the increase in the mortality risk (Galor & Moav, 2007).
: Life Expectancy: From the Neolithic Period to the Iron Age table
3.3: Population Growth The first period in pre-modern times, is Palaeolithic period that lasted 3 to 5 million years. Subsequent period, however, are indicated according to scale, starting with the Neolithic period that concerned about 8000 B.C. and lasted until about 3000 B.C. This was followed by the Bronze ages, which in turn were followed by the middles ages. The middle ages continued until the modern era that began around the seventh century A.D. The growth of populations in pre- modern times was very slow. Although reproduction was almost optimal in those times, the extremely difficult circumstances under which people lived led to almost as much deaths as births, with the result that populations did not show significant growth. The slight growth that did occur was off-set from time to time by natural disasters, epidemics famines and wars. Although, a change from nomadic to a more sedentary way of life in countries such as Mesopotamia, Egypt, Crete, India, China and Peru resulted in the establishment of well-ordered communities, it did not lead to a notable acceleration in the growth of populations. With the establishment of farming communities at the beginning of the Neolithic period, the world population started to grow slowly-from an estimated 5 million to more or less 10 million around 8000 B.C more or less 300 million at the beginning of our era, and more or less 500 million in 1650 (Mostert, 1998). Population growth has recently become a central issue in many archaeological models of culture change, yet only a few studies have ad dressed the problem of determining population growth rates. Researchers provided an estimate of 0.1% for annual growth rate during the Neolithic on the basis of a guess-estimate of the number of the inhabitants of the Near East at the beginning of the Neolithic (50,000 100,000 persons) and about 1-12 million persons at about 4000 B.C. These figures indicate a probable rate of population growth between 0.08 and 0.12% per year. A lower rate of 0.03% was estimated for the period preceding the Neolithic and 0.024% for the period immediately following the Neolithic. The average for the Neolithic period was estimated at 0.1%. A report provided an average of less than 0.1% from 8000 B.C. to about A.D. 1500. The average rate of population growth during the Palaeolithic is exceeding in comparison with that of the Neolithic; and it has been found that until A.D. 1700 the human population grew at an average rate of 0.002% per year. The rise of civilizations seems to have been associated with relatively rapid rates of population increase (Hassan, 1979).
World Population Growth from Early Agriculture to Modern Era image
3.4: Fertility: In demographic terminology fertility refers to the number of children born to an individual, whereas fecundity refers to the physiological ability to bear children. As with mortality, natural patterns of fertility characterize human and animal populations. However, whereas mortality has the potential to affect all sectors of a population, births are confined to those individuals who are fecund. Furthermore, human fertility (to a much greater extent than mortality) is directly influenced by individual reproductive behaviour, so that parameters such as age at first marriage and deliberate birth spacing have strong effects on population fertility rates (Chamberlain, 2006). From an updated sample of Old and New World cemeteries in the northern hemisphere a birth rate estimate is obtained ranging from 40.3 to 53.1 per 1000, over 1000 years during the Neolithic fertility transition (Bocquet-Appel, 2008).
4. Summary Population growth after the development of agriculture has been one of the most dramatic features of human evolution and estimated population was (10,000 years ago) 8,000,000 individuals who inhabited the earth. By A.D. 1800, the world’s population reached 1,000,000,000 souls. Male and female adult skeletal morphology varies across a continuum. Male and female adult skeletal morphology varies across a continuum. Sex and age can be determined from skeleton, which can be later helpful in assessing the number of skeleton, age at death, fertility and mortality ratios and population growth patterns. Data from ancient populations, Europe and Asia, show that only a small proportion of the population lived beyond 50, most died at about sixty or seventy years old. In the case of the Middle East (Southern Levant), Hershkovitz and Gopher (2008), studied hundreds of skeletons that came from a hunter gatherer series of Natufian period (15,000 to 12,000 a. P.) and preceramic sedentary groups from the Neolithic (from 12,000 to 8350 a. P). For the former the estimated life expectancy at birth, assuming they are stationary population, is 24.6 years, while for the latter it is 25.4 years. Paleodemographers, however, have typically reported unexpectedly low infant mortality and adult age specific mortality that is abnormally high and that accelerates rapidly from age 15 to 50, resulting in few individuals surviving into old age. The rise in population density, the domestication of animals, and the increase in work effort in the course of the Neolithic Revolution increased the exposure and the vulnerability of humans to environmental hazards, such as infectious diseases, and led to the decline in life expectancy during that period. The first period in pre-modern times, is Palaeolithic period that lasted 3 to 5 million years. Subsequent period, however, are indicated according to scale, starting with the Neolithic period that concerned about 8000 B.C. and lasted until about 3000 B.C. This was followed by the Bronze ages, which in turn were followed by the middles ages. Furthermore, human fertility (to a much greater extent than mortality) is directly influenced by individual reproductive behaviour, so that parameters such as age at first marriage and deliberate birth spacing have strong effects on population fertility rates. From an updated sample of Old and New World cemeteries in the northern hemisphere a birth rate estimate is obtained ranging from 40.3 to 53.1 per 1000, over 1000 years.