17 Skeletal Analysis and Determination of Demographic Variables in Prehistoric Populations

Dr. Vijeta Dr. Vijeta

1. Introduction

The human skeleton proves to be an extremely valuable source for the reconstruction of past life parameters due to its resistance to decomposition. Archaeologists, historians, and anthropologists alike rely on these biological building blocks for many palaeo-demographic inferences and, not surprisingly, there is a longstanding tradition of establishing mortality profiles from prehistoric cemetery populations. These death structures serve as indicators of overall life expectancy, fertility, and even population growth. Moreover, historical patterns of health, disease, and ontogenesis are used to isolate biological as well as social life history factors. (Hoppa & Vaupel, 2008). Mean age at death of this population was about 35 years with some individuals having survived till 50 years. The prehistoric sample displays complex biological variation and evolution of prehistoric inhabitants, possessed general skeletal biology, facilitates synthetic review of biological temporal trends, and suggests a general temporal increase in certain pathological conditions and other indicators of population stress (Seth, 2003).

2.    Skeletal analysis

The demographic study of a skeletal sample begins with the assessment of both the age at death and sex distribution of the skeletal material. These data are absolutely essential for the construction of other population statistics.

2.1: Assessment of Age: For the fetus and the young infant, forensic dentists, through the utilization of X-rays can reasonably estimate age from 13 to 16 weeks in utero through 14 years of age, when calcification of the permanent second molar usually occurs. An understanding of the time of appearance of major ossification centers is very helpful in the determination of skeletal age in the child and young adult. The appearance of ossification centers is typically completed around 5 years of age. From the period of 5 years of age up to approximately 25 years of age the fusion of the epiphyses is utilized for age determination. When determining the age of skeletal remains, important points are:

1. The age of skeletal remains are affected by sex, race and nutrition of the deceased. Also, maturity of the deceased is not synonymous with the calendar age.
2. Female skeletal remains are almost always in advance of males of the same age, race, nutrition level and geographic location. Maturity tends to be more advanced in those who live in the hotter climates.
3. There are substantive variations in epiphyseal closure dates. Epiphyseal union occurs over a period of time and not all at once. As an example, epiphyseal union of the medial end of the clavicle occurs typically over a period of time ranging from 18 to 30 years.

Examination of epiphyseal closure of long bones is used for the determination of the age of skeletal remains extending from the mid-teens to the early twenties. Skeletal aging in the later years is not nearly as finite as skeletal aging in the child and the young adult. From approximately 25 years until old age, there are no dramatic events such as tooth eruption or the appearance of ossification centers. There are three anatomical structures which are used in this period of life for aging, the pubic symphysis, sternal ribs and skull sutures. The right and left hip-bones (innominate bones) meet in the midline in front to form the pubic symphysis. It should be understood the right and left pubic bones do not actually articulate: they are separated throughout life by the symphyseal cartilage. Each pubic bone presents a symphyseal surface or face (https://forensicmd.files.wordpress.com).

2.1.1: Age determination through Skull: Age determination can be accomplished through as follows-

• Metopic suture closes at 2 to 4 years
• Sagittal suture– 30 to 40 years
• Coronal suture – 40 to 50 years
• Lambdoid suture – 40 to 50 years
• Sphenotemporal suture – 50 to 60 years
• Parietomastoid suture – 80 to 90 years
• Masto-occipital suture – 80 to 90 years
• Squamous suture – above 80 years
• Basal suture fuses by 18 to 20 years

Fig. I: Parts of human skull with sutures

(Source Link: www.googleimage.com)

Secondary Changes in Skull: Texture –the texture of a young adult skull is smooth and ivorine on both surfaces. At about 40 ± 5 years, the skull surface begins to assume “matted, granular and rough appearance”. Markings on skull – after the age of 25 year onwards the muscular markings become increasingly evident. The markings are – temporal line, nuchal lines, and masseteric attachment on side of mandible. After 50 years, the diploe becomes less vascular with increasing replacement by bone. The grooves for middle meningeal artery become deeper. Thickness of skull – (Todd, 1924) the thickness of skull increases with age. The increase in thickness is more after 50 years up to 60 years with no decrease thereafter. Increase in skull size on lateral skull radiograph, Israel noted increase in skull size with increase in skull thickness and skull diameter with advancing age (Bardale, 2011).

2.1.2: Age determination through Tooth structure:

(Source Link: www.googleimage.co.in)

2.1.3: Age determination through Height and Weight: Measurement of height gives idea regarding the development of an individual. Average height is term employed which means height of a person within range of permissible limits i.e. in other words; it lies between third and ninety-seventh percentiles of height or within two standard deviations above or below the mean height of the age. The length of child at birth is about 50 cm, 60 cm at 3 months, 70 cm at 9 month and 73 to 75 cm at 1 year of extra uterine life. At age of 2 years, the height is about 90 cm and at 4.5 years, the height is about 100 cm. After that the child gains about 5 cm in height every year up to 10 years of age. During the onset of puberty, there occurs growth spurt and add about 20 cm in height of a person. Thus, there is about 20 cm net increment in height of a person at puberty (Bardale, 2011).

2.2: Assessment of Sex from Skull:

(Source Link: https://forensicmd.files.wordpress.com/2010/11/identification-of-skeletal-remains.pdf)

2.2.1: Assessment of Sex from Pelvis, Hip Bone and Sacrum:

The best bones for the determination of sex are those of the pelvis, which has an accuracy of 98% when properly examined. The pelvic bone (innominate) is composed of three bones, the pubic, ischial and iliac. Of these three bones it is the pubic that is best utilized for the determination of sex.

The pelvis as a whole in the male is more massive and has prominent muscle ridges. In the female the pelvis is more slender, smaller and relatively smooth. The sub pubic angle in the male is V-shaped with an angle less than 90 degrees, usually about 70 degrees (upper image), whereas the supubic angle in the female is U-shaped, rounded, broader, divergent with an angle of 90 degrees or higher (lower image). The body of the pubic bone, which is lateral to the symphysis, tends to be triangular in males, whereas in females it is more rectangular; in females there is a bony ridge running down the ventral surface from the pubic crest; in females there is a concavity of the lower margin of the inferior pubic ramus immediately lateral to the lower border of the symphysis; in females there is a ridge of elevated bone on the medial aspect of the ischiopubic ramus, immediately lateral to the symphysis, whereas in males this area is broad and flat; the ischiopubic index (pubic length x 100 divided by the ischial length is less than 90 in adult white males, in adult females it is over 95). The greater sciatic notch in the male is smaller, deeper and narrow typically at an angle of less than 68 degrees, whereas in the female it is larger and more open, divergent, typically at an angle of 68 degrees or greater. The obturator foramen is more ovoid in the male, but triangular in the female; the pre-auricular sulcus, which serves for the attachment of the anterior sacroiliac ligament, lies just lateral to the sacroiliac joint and is well defined in females, but virtually absent in males. The sacrum in the male is typically longer, narrower and has a more evenly distributed continuous curve down the whole bone, sometimes with a slight forward projection of the coccyx as compared to the female. In females the sacrum is shorter, broader with a prominent curve between S1-2 and S3-5. Also, the superior articular surface in males is larger than that of females. The male sacrum may have more than five segments, which is rare in the female. The pelvic cavity in males is relatively narrow and deep, whereas in females it is wide and shallow. The length of the ilium is greater than its height making it to appear more vertical in the male, whereas in the female the ilium appears to be lower and to flare outward. The pubic symphysis is higher in the males as compared to the female.

Determination of Sex from other Bones: Other bones that can be used to determine sex are the sternum, scapula and femur. The sternum is divided into two major parts, the upper half, the manubrium and the lower, the body, which comprises much of the sternum. The body in males is at least twice the length of the manubrium. It is less than this in females. The scapula in males will generally have a deep supra-scapular notch. That of the female is shallower. The vertical diameter of the glenoid cavity in females is less than 36 mm, in males it is greater. Of the long bones, it is the femur, which is most commonly used for sex determination. The femur in males is larger and heavier than in females. The angle of the neck to the shaft is greater in females. Lastly, if the head diameter is less than 46 mm it is generally female. The angle formed by the neck of the femur with its shaft (the collodiaphyseal angle) is less than 40 degrees in the male and greater than 50 degrees in the female (https://forensicmd.files.wordpress.com).

3.    Demography in Pre-Historic Population

The likely approximate size of this first human population was 100,000, probably already divided into distinct groups. They emerged just as the Earth was entering an ice age, which, like any climatic change, resulted in the disappearance of many species and the emergence of new ones. Homo Habilis disappeared as Homo Ergaster emerged, spreading throughout Africa, and thence across Europe and Asia in the form close to Homo erectus. The standard method for estimating prehistoric population distribution is to attribute to a given area the population density recorded in a recent period among people of a similar culture living in a similar environment and climate. In the Palaeolithic era, population distribution was much more closely linked to the size of the territory populated and to climatic variations than to the still very primitive technology. It therefore remained very sparse despite odd technical leaps like learning to control fire. The total numbers of Homo Ergaster and Homo erectus may be very roughly estimated at between 500,000 and 700,000 in the old world (Eurasia and Africa), which were the only populated areas at the time. Then, between 300 000 and 200 000 BCE (Before Christian/Common Era), three distinct, hominid population groups developed at the same time but far apart, separated by the oceanic rise in the last two interglacial periods: modern man (Homo sapiens) in Africa and Southern Asia (perhaps 800,000 individuals), Neanderthal man in Europe (perhaps 250,000), and Java man in Indonesia (perhaps 100,000). With the last ice flood, around 70 000 BCE, falling sea levels brought the three hominid population groups into contact. Homo sapiens asserted his supremacy everywhere, squeezing out first Java man, then Neanderthal man, and spreading between 50,000 and 40,000 BCE across the as yet unpopulated continental land masses: Australia, the two Americas and later on, Siberia. The world population at this time may have totalled 1.5 million, including 1 million in Africa and Asia, 50,000 in Australia, 300,000 in America and 150,000 in Europe, the latter two continents being still largely under ice. Around 40,000 BCE, technological progress in the form of the invention of the spear-thrower, the harpoon, the bow and arrow vastly improved the efficiency of hunting and fishing, and became the main engine of population growth, especially in Europe. Taking advantage of the falling sea level, which greatly narrowed the strait between Sicily and Tunisia, two waves of Europeans migrated to North Africa, around 20,000 BCE and 12,000 BCE. They populated it from the Canaries to Egypt, stretching even as far as Arabia. At the height of the Late Palaeolithic Age, from 10,000 to 9,000 BCE, the population of Europe may have stood at 200,000 people. The sudden climatic warming which occurred around 8,650 BCE halted their growth, and the beginning of the Mesolithic era saw the population decrease then increase rapidly with the cultural adaptation to the new climate and the repopulation of Northern Europe as the ice melted. Around 7000 BCE, it is likely that Europe had close to 400,000 inhabitants. With the Neolithic era in the Middle East—from 10,000 to 8,000 BCE—came sedentariness, hand-hoe cropping, stock-rearing, pottery-making and navigation, resulting in a tenfold increase in the population from 0.5 to 5 million inhabitants. From Anatolia, Neolithic peoples migrated to Greece, settling near what would become Thessaloniki, and from this densely- populated settlement sent out two streams that propagated Neolithic culture in Europe: one sea-borne, investing the coastal regions as far as England, the other across land, moving up the Danube to occupy the central part of the continent. By around 4,000 BCE, the Neolithic culture had spread across Europe, with a population of perhaps 2 million, rising so rapidly that it could well have topped 23 million by around 2,000 BCE, when the advent of the Bronze Age brought a population decline. In Indian context, Neolithic culture first emerged there in the Punjab, which also rapidly developed into a major population centre, rising from perhaps 0.7 to 20 million between 4,000 and 2,000 BCE. From 8,000 BCE, a Neolithic culture also developed in the Huang Ho river basin (China), extending towards the east, then the south where corn gave way to rice. Here, again, the population rose from 0.8 to 20 million between 4,000 and 2,000 BCE. Other Neolithic civilizations developed somewhat later in Mexico and on the high plateaux of the Andes, likewise bringing a population surge. Finally other partial civilizations developed around pottery and primitive farming from 12,000 BCE in Japan and 8,500 BCE in the African Sahel (Biraben, 2003).

3.1: Population Size: A disparate range of archaeological evidence, including the number and sizes of houses within settlements, the areal extent of settlements, the economic potential of the catchment areas around population centers, and various measures of the exploitation, consumption, and discard of raw materials and artifacts, can be used as proxies for estimating population size and density. For example, Bocquet-Appel et al. (2005) modeled population sizes in Upper Paleolithic Europe using the spatial density of archaeological sites (a proxy measure of population density) combined with numerical estimates of population density taken from ethnographic studies of North American foragers who lived under similar bioclimatic conditions to those experienced in Europe during the late glacial period. By assuming that the average population density derived from the ethnographic data represented the carrying capacity of the late glacial environments i.e., the maximum population density for the European late glacial period, they were able to convert their archaeological data into estimates of actual population density from which population size and growth over time could be modeled. Measures of relative population size based on artifact discard rates have also been used to determine correlations between large-scale environmental changes and hominin population density during the middle and upper Pleistocene in Britain. Hosfield (1999, 2005) and Ashton and Lewis (2002) identified relative changes in human population density in late middle Pleistocene Britain by quantifying the density of accumulations of bifacial stone tools in gravel terraces in southern Britain from oxygen isotope stages (OISs) 13 to 2, a period of approximately 500,000 years. Their calculations take into account the spatial extent of commercial minerals extraction and urban development in their study areas (two of the principal factors affecting the archaeological data through their influence on the discovery potential of stone artifacts). The results of these studies indicate a sharp decline in evidence for human activity in the Middle Thames valley after OIS 10 (350,000 years ago), although 100 km to the south, in the area surrounding the former Solent River, the population appeared to increase during late OIS 9/early OIS 8, suggesting some regional variations in population processes during this time period.

Population Growth: The increasingly large data sets compiled from radio- carbon dating programs provide an index of changes through time in human population density, an approach that has been used to ascertain the timing of the extinction of Neanderthal populations and the subsequent colonization and recolonization of Europe by modern humans during the late Pleistocene. A useful application of spatial databases of radiocarbon dates is to determine the rates of spread of demographic waves of advance during continental-scale periods of colonization and cultural change. The proxy data on population numbers provided by radiocarbon dating can be combined with estimates of fertility and migration in the construction of colonization models. The rates of advance of Paleolithic hunter-gatherer populations reentering northwest Europe after the last glacial maximum have been estimated to be 0.5–2 km/yr, values that are similar to those established for the spread of early farming; the higher migration rates of foragers presumably compensate for their lower fertility and longer intergeneration intervals, which would otherwise slow demographic expansion rates (Chamberlain, 2009).

3.2: Age at Death Distributions and Mortality Patterns: The levels and age distributions of mortality (i.e., mortality profiles) for past populations have been reconstructed through the application of life table methods and/or hazards modelling to age at death distributions estimated from assemblages of human skeletal remains. Such approaches rely on the use of skeletal indicators of growth and senescence that show consistent correlations with age at death across samples and populations. Traditional anthropological methods for estimating age at death from the skeleton have typically used measures of the central tendency of age for particular skeletal indicators, or these methods have relied on regressions of age on skeletal indicator state (so-called inverse regressions) to predict age at death in cemetery populations. The short term potential for human population growth in small populations is often high, with instantaneous population growth rates of between 0.5% and 2% per year documented for hunter-gatherer groups such as the Ache, Agta, Asmat, Hadza and Yanomama. However, estimates of long-term population growth rates based on historical and archaeological data are typically much closer to 0, suggesting that episodes of catastrophic mortality that cause substantial losses to living populations and that occur every few generations may account for the balance between short-term and long-term population growth.

Fertility: Unlike mortality (which frequently leaves an archaeological signature in the form of skeletal remains accompanied by evidence of funerary ritual), fertility is much less visible in the archaeological record and estimates of fertility are usually derived indirectly from measures of mortality and population growth. The material traces of the birth process are ephemeral in the archaeological record; birth events do not often result in a recognizable depositional event, and neonatal mortality especially is often underrepresented in mortuary assemblages. Variations in fertility can be investigated indirectly through their effects on the age distribution of deaths, and a simple paleodemographic measure that is responsive to fertility is the juvenility index, in which the numbers of deaths of older children are expressed as a ratio of their deaths to the number of adult deaths in the population .The deaths of infants and younger children are excluded from the calculation of the juvenility index to avoid the biasing effects of differential mortuary practices and post depositional preservation potential that adversely affect the proportions of these age categories in skeletal samples. As fertility is closely correlated with overall mortality, the juvenility index is regarded as a suitable proxy for estimating fertility in past populations. In communities practicing agriculture, female total fertility is on average higher than that in foraging populations, with age-specific female fertility peaking in the early 20s rather than in the late 20s or early 30s, as observed in foraging populations, and annual birth rates are generally higher at all maternal ages in agriculturalists.

Life Span: A key challenge for archaeological demographers, especially for those studying prehistoric populations, is to determine the extent to which uniformitarian models can be applied in paleodemography. From research on human life history variables it is apparent that milestones in both development and senescence, such as age at weaning, age at reproductive maturation, the timing of female fertility decline, and maximum potential life span, are subject to stabilizing selection and are relatively invariant across present-day human population. Some life history variables (e.g., fertility and longevity) are difficult to measure in past populations, but one life-history-related feature of skeletal development that provides a reliable chronological marker in both extant and fossil species is the time taken for the crowns of the teeth to form before the teeth erupt into the mouth. Studies of the chronology of tooth development based on the counting of incremental growth markers in dental enamel have shown that fossil hominin species achieved dental maturity in approximately two-thirds the time that modern humans require to reach an equivalent developmental stage; in addition, modern human tooth formation times appear to have been established more than 150,000 years ago in the earliest representatives of anatomically modern Homo sapiens. Because life history variables are strongly inter-correlated, at least at the level of species comparisons, the distinctive pattern of delayed maturation that is characteristic of anatomically modern Homo sapiens is expected to be accompanied by increased longevity and maximum life span. The estimation of longevity in fossil hominins is not straightforward because, unlike the situation with enamel growth, adult age at death is not directly measurable and must be inferred from skeletal indicators that have only an imprecise correlation with the chronological age of the individual. Maximum potential life span in great apes appears to be between 50–60 years (wild populations) and 60 years (captive animals), with wild individuals exhibiting physical and behavioral signs of senescence from their mid-30s onward. At some stage in the human evolutionary lineage a delayed onset of senescence and an extension of longevity appear to have evolved, perhaps concurrently with the extended period of maturation evident from the record of dental development. Caspari and Lee (2004) investigated the evolution of human longevity by calculating the ratio of older to younger adults in samples of fossil hominins belonging to the genera Australopithecus and Homo. Older adults were defined as those individuals estimated from dental wear to be more than twice the age at which skeletal maturity was achieved and determined that the proportion of older adult individuals in their fossil species samples increased from 10% in Australopithecus to 20% in early Homo and 28% in Homo neanderthalensis. In contrast, a much higher proportion of older adults (approximating contemporary modern human values of about 70%) was found in their sample of European early Upper Paleolithic Homo sapiens.

Life Tables: First, model life tables calculated for wild populations of chimpanzees (Pan troglodytes) show that these apes have attritional mortality patterns in which most adult deaths fall into the “older adult” category. The Pliocene-Pleistocene assemblage of Homo habilis from Olduvai Gorge, the late Pleistocene sample of Homo heidelbergensis from Sima de le Huesos in Spain and the Homo neanderthalensis specimens from Krapina in Croatia are all dominated by high proportions of adolescent individuals (30–64% of these samples). Adolescents constitute the age category least expected to be present in attritional mortality assemblages because they represent the age at which risk of death is minimized in model life tables. The presence of adolescents in the hominin mortuary assemblages is actually a signature of catastrophic mortality, as the adolescent age category forms a substantial proportion of the living population. The distinctive hominin pattern of excess adolescent and young adult mortality is predicted if predation (either by large carnivores in the case of early hominin species or through inter- and intraspecific violence in the case of later species of Homo) made a significant contribution to the formation of the fossil hominin assemblages. Both large carnivores and human hunter-gatherers commonly use hunting methods that select prey in proportion to encounter rates, a practice that generates age distributions in the prey assemblages that mirror the living age structure of the prey population (Chamberlain, 2009).

4: Summary

Due to its resistance to decomposition, the human skeleton proves to be an extremely valuable source for the reconstruction of past life parameters. The prehistoric sample displays complex biological variation and evolution of prehistoric inhabitants, possessed general skeletal biology, facilitates synthetic review of biological temporal trends, and suggests a general temporal increase in certain pathological conditions and other indicators of population stress. Due to its resistance to decomposition, the human skeleton proves to be an extremely valuable source for the reconstruction of past life parameters. These death structures serve as indicators of overall life expectancy, fertility, and even population growth. Moreover, historical patterns of health, disease, and ontogenesis are used to isolate biological as well as social life history factors. A demographic study of a skeletal sample begins with the assessment of both the age at death and sex distribution of the skeletal material. A disparate range of archaeological evidence, including the number and sizes of houses within settlements, the areal extent of settlements, the economic potential of the catchment areas around population centers, and various measures of the exploitation, consumption, and discard of raw materials and artifacts, can be used as proxies for estimating population size and density. The distinctive hominin pattern of excess adolescent and young adult mortality is predicted if predation either by large carnivores in the case of early hominin species or through inter- and intraspecific violence in the case of later species of Homo made a significant contribution to the formation of the fossil hominin assemblages.

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