Paleodemography: “Not quite dead”

Estimating fertility using adults: A method for under-enumerated pre-adult skeletal samples

OBJECTIVES

Infant underrepresentation poses a great risk to accurate palaeodemographic findings when analyzing skeletal samples. Empirically derived palaeodemographic methods all require unbiased or minimally biased pre-adult representation for estimating demographic characteristics, including fertility. Currently, there are no reliable methods for estimating palaeodemographic parameters when pre-adults are underrepresented in skeletal samples, consequently such samples are often excluded from palaeodemographic analyses. The aim of this article is to develop a method for estimating total fertility rate (TFR) using reproductive aged adults, specifically for samples with suspected pre-adult under-enumeration.

METHODOLOGY

United Nations mortality data and TFR from the World Population Prospects was utilized. The correlation between known fertility and the proportion of individuals in key reproductive years (15-49 years) to total adult sample (15+ years) was assessed as an indirect means to estimate fertility.

RESULTS

It was determined that the proportion of reproductive aged adults is a reasonable proxy for fertility. A significant positive correlation was observed between the TFR and those who died aged 15-49 years of age as a proportion of those who died ≥15 years (D15-49/D15+). SE of the estimate revealed reasonable predictive accuracy. When applied to two modern non-agricultural populations, the method showed some variability in accuracy but good potential for an improved outcome over existing methods when pre-adults are underrepresented.

CONCLUSION

This research has provided a new method for estimating fertility in archeological skeletal samples with pre-adult under-enumeration. In combination with a contextually focused approach, this provides a significant step toward further use of biased samples in palaeodemography.

Twenty-first century bioarchaeology: Taking stock and moving forward

This article presents outcomes from a Workshop entitled "Bioarchaeology: Taking Stock and Moving Forward," which was held at Arizona State University (ASU) on March 6-8, 2020. Funded by the National Science Foundation (NSF), the School of Human Evolution and Social Change (ASU), and the Center for Bioarchaeological Research (CBR, ASU), the Workshop's overall goal was to explore reasons why research proposals submitted by bioarchaeologists, both graduate students and established scholars, fared disproportionately poorly within recent NSF Anthropology Program competitions and to offer advice for increasing success. Therefore, this Workshop comprised 43 international scholars and four advanced graduate students with a history of successful grant acquisition, primarily from the United States. Ultimately, we focused on two related aims: (1) best practices for improving research designs and training and (2) evaluating topics of contemporary significance that reverberate through history and beyond as promising trajectories for bioarchaeological research. Among the former were contextual grounding, research question/hypothesis generation, statistical procedures appropriate for small samples and mixed qualitative/quantitative data, the salience of Bayesian methods, and training program content. Topical foci included ethics, social inequality, identity (including intersectionality), climate change, migration, violence, epidemic disease, adaptability/plasticity, the osteological paradox, and the developmental origins of health and disease. Given the profound changes required globally to address decolonization in the 21st century, this concern also entered many formal and informal discussions.

Paleodemography: From archaeology and skeletal age estimation to life in the past

Much of paleodemography, an interdisciplinary field with strong ties to archaeology, among other disciplines, is oriented toward clarifying the life experiences of past people and why they changed over time. We focus on how human skeletons contribute to our understanding of preindustrial demographic regimes, including when changes took place that led to the world as we know it today. Problems with existing paleodemographic practices are highlighted, as are promising directions for future work. The latter requires both better age estimates and innovative methods to handle data appropriately. Age-at-death estimates for adult skeletons are a particular problem, especially for adults over 50 years that undoubtedly are mistakenly underrepresented in published studies of archaeological skeletons. Better age estimates for the entirety of the lifespan are essential to generate realistic distributions of age at death. There are currently encouraging signs that after about a half-century of intensive, and sometimes contentious, research, paleodemography is poised to contribute much to understandings of evolutionary processes, the structure of past populations, and human-disease interaction, among other topics.

What is the expected human childhood? Insights from evolutionary anthropology

In psychological research, there are often assumptions about the conditions that children expect to encounter during their development. These assumptions shape prevailing ideas about the experiences that children are capable of adjusting to, and whether their responses are viewed as impairments or adaptations. Specifically, the expected childhood is often depicted as nurturing and safe, and characterized by high levels of caregiver investment. Here, we synthesize evidence from history, anthropology, and primatology to challenge this view. We integrate the findings of systematic reviews, meta-analyses, and cross-cultural investigations on three forms of threat (infanticide, violent conflict, and predation) and three forms of deprivation (social, cognitive, and nutritional) that children have faced throughout human evolution. Our results show that mean levels of threat and deprivation were higher than is typical in industrialized societies, and that our species has experienced much variation in the levels of these adversities across space and time. These conditions likely favored a high degree of phenotypic plasticity, or the ability to tailor development to different conditions. This body of evidence has implications for recognizing developmental adaptations to adversity, for cultural variation in responses to adverse experiences, and for definitions of adversity and deprivation as deviation from the expected human childhood.

Paleodemographic analysis of age at death for a population of Black Sea Scythians: An exploration by using Bayesian methods

OBJECTIVES

Studies of the demography of past populations involving deterministic life tables can be criticized for ignoring the errors of estimation. Bayesian methods offer an alternative, by focusing on the uncertainty of the estimates, although their results are often sensitive to the choice of prior distributions. The aim of this study is to explore a range of Bayesian methods for estimating age at death for a population of nomadic warriors-Scythians from the Black Sea region.

MATERIALS AND METHODS

In total, skeletons of 312 individuals (93 children and 219 adults) from Glinoe (Moldova), dated to the 5th-2nd century BCE, were examined. We unified the age categories corresponding to different aging methods, allowing an application of a probabilistic assessment of the age categorization. A hierarchical Bayesian multinomial-Dirichlet-Dirichlet model was applied, with a hypothetical, subjective reference population, a real reference population, and no reference.

RESULTS

Stationary-population life expectancy was estimated as 27.7 years (95% CI: 25.1-30.3) for a newborn (e₀ ), and 16.4 years (14.0-19.0) for 20-year-olds (e₂₀ ), although with high uncertainty, and sensitive to the model specification. Slight differences in longevity between different social strata and between the Classical and Late chronological periods were found, although with high estimation errors. A more robust finding, confirming earlier studies, was a high probability of death in young adulthood, which could depend on Scythian lifestyle (conflicts, wars).

DISCUSSION

Our study shows a way to overcome some limitations of broad age categorization by using the Bayesian approach with alternative model specifications, allowing to assess the impact of reference populations.

Demographic uniformitarianism: the theoretical basis of prehistoric demographic research and its cross-disciplinary challenges

A principle of demographic uniformitarianism underpins all research into prehistoric demography (palaeodemography). This principle-which argues for continuity in the evolved mechanisms underlying modern human demographic processes and their response to environmental stimuli between past and present-provides the cross-disciplinary basis for palaeodemographic reconstruction and analysis. Prompted by the recent growth and interest in the field of prehistoric demography, this paper reviews the principle of demographic uniformitarianism, evaluates how it relates to two key debates in palaeodemographic research and seeks to delimit its range of applicability to past human and hominin populations. This article is part of the theme issue 'Cross-disciplinary approaches to prehistoric demography'.

A comparison of subadult skeletal and dental development based on living and deceased samples

OBJECTIVES

A fundamental assumption in biological anthropology is that living individuals will present with different growth than non-survivors of the same population. The aim is to address the question of whether growth and development data of non-survivors are reflective of the biological consequences of selective mortality and/or stress.

MATERIALS AND METHODS

The study compares dental development and skeletal growth collected from radiographic images of contemporary samples of living and deceased individuals from the United States (birth to 20 years) and South Africa (birth to 12 years). Further evaluation of deceased individuals is used to explore differential patterns among manners of death (MOD).

RESULTS

Results do not show any significant differences in skeletal growth or dental development between living and deceased individuals. However, in the South African deceased sample the youngest individuals exhibited substantially smaller diaphyseal lengths than the living sample, but by 2 years of age the differences were negligible. In the US sample, neither significant nor substantial differences were found in dental development or diaphyseal length according to MOD and age (>2 years of age), though some long bones in individuals <2 years of age did show significant differences. No significant differences were noted in diaphyseal length according to MOD and age in the SA sample.

DISCUSSION

The current findings refute the idea that contemporary deceased and living individuals would present with differential growth and development patterns through all of ontogeny as well as the assumptions linking short stature, poor environments, and MOD.

Bayesian Statistics in Archaeology

Null hypothesis significance testing (NHST) is the most common statistical framework used by scientists, including archaeologists. Owing to increasing dissatisfaction, however, Bayesian inference has become an alternative to these methods. In this article, we review the application of Bayesian statistics to archaeology. We begin with a simple example to demonstrate the differences in applying NHST and Bayesian inference to an archaeological problem. Next, we formally define NHST and Bayesian inference, provide a brief historical overview of their development, and discuss the advantages and limitations of each method. A review of Bayesian inference and archaeology follows, highlighting the applications of Bayesian methods to chronological, bioarchaeological, zooarchaeological, ceramic, lithic, and spatial analyses. We close by considering the future applications of Bayesian statistics to archaeological research.

Life not death: Epidemiology from skeletons

Analytically sophisticated paleoepidemiology is a relatively new development in the characterization of past life experiences. It is based on sound paleopathological observations, accurate age-at-death estimates, an explicit engagement with the nature of mortality samples, and analytical procedures that owe much to epidemiology. Of foremost importance is an emphasis on people, not skeletons. Transforming information gleaned from the dead, a biased sample of individuals who were once alive at each age, into a form that is informative about past life experiences has been a major challenge for bioarchaeologists, but recent work shows it can be done. The further development of paleoepidemiology includes essential contributions from paleopathology, archaeology or history (as appropriate), and epidemiology.

Maximum likelihood estimate of life expectancy in the prehistoric Jomon: Canine pulp volume reduction suggests a longer life expectancy than previously thought

Recent theoretical progress potentially refutes past claims that paleodemographic estimations are flawed by statistical problems, including age mimicry and sample bias due to differential preservation. The life expectancy at age 15 of the Jomon period prehistoric populace in Japan was initially estimated to have been ∼16 years while a more recent analysis suggested 31.5 years. In this study, we provide alternative results based on a new methodology. The material comprises 234 mandibular canines from Jomon period skeletal remains and a reference sample of 363 mandibular canines of recent-modern Japanese. Dental pulp reduction is used as the age-indicator, which because of tooth durability is presumed to minimize the effect of differential preservation. Maximum likelihood estimation, which theoretically avoids age mimicry, was applied. Our methods also adjusted for the known pulp volume reduction rate among recent-modern Japanese to provide a better fit for observations in the Jomon period sample. Without adjustment for the known rate in pulp volume reduction, estimates of Jomon life expectancy at age 15 were dubiously long. However, when the rate was adjusted, the estimate results in a value that falls within the range of modern hunter-gatherers, with significantly better fit to the observations. The rate-adjusted result of 32.2 years more likely represents the true life expectancy of the Jomon people at age 15, than the result without adjustment. Considering ∼7% rate of antemortem loss of the mandibular canine observed in our Jomon period sample, actual life expectancy at age 15 may have been as high as ∼35.3 years.

Mechanical and metabolic interactions in cortical bone development

OBJECTIVES

Anthropological studies of cortical bone often aim to reconstruct either habitual activities or health of past populations. During development, mechanical loading and metabolism simultaneously shape cortical bone structure; yet, few studies have investigated how these factors interact. Understanding their relative morphological effects is essential for assessing human behavior from skeletal samples, as previous studies have suggested that interaction effects may influence the interpretation from cortical structure of physical activity or metabolic status.

MATERIAL AND METHODS

This study assesses cross-sectional geometric and histomorphometric features in bones under different loading regimes (femur, humerus, rib) and compares these properties among individuals under different degrees of metabolic stress. The study sample consists of immature humans from a late medieval Lithuanian cemetery (Alytus, 14th-18th centuries AD). Analyses are based on the hypothesis that metabolic bone loss is distributed within the skeleton in a way that optimizes mechanical competency.

RESULTS

Results suggest mechanical compensation for metabolic bone loss in the cross-sectional properties of all three bones (especially ribs), suggesting a mechanism for conserving adequate bone strength for different loads across the skeleton. Microscopic bone loss is restricted to stronger bones under high loads, which may mitigate fracture risk in areas of the skeleton that are more resistive to loading, although alternative explanations are examined.

DISCUSSION

Distributions of metabolic bone loss and subsequent structural adjustments appear to preserve strength. Nevertheless, both mechanics and metabolism have a detectable influence on morphology, and potential implications for behavioral interpretations in bioculturally stressed samples due to this interaction are explored. Am J Phys Anthropol 160:317-333, 2016. © 2016 Wiley Periodicals, Inc.

An informative prior probability distribution of the gompertz parameters for bayesian approaches in paleodemography

OBJECTIVES

In paleodemography, the Bayesian approach has been suggested to provide an effective means by which mortality profiles of past populations can be adequately estimated, and thus avoid problems of "age-mimicry" inherent in conventional approaches. In this study, we propose an application of the Gompertz model using an "informative" prior probability distribution by revising a recent example of the Bayesian approach based on an "uninformative" distribution.

METHODS

Life-table data of 134 human populations including those of contemporary hunter-gatherers were used to determine the Gompertz parameters of each population. In each population, we used both raw life-table data and the Gompertz parameters to calculate some demographic values such as the mean life-span, to confirm representativeness of the model. Then, the correlation between the two Gompertz parameters (the Strehler-Mildvan correlation) was re-established. We incorporated the correlation into the Bayesian approach as an "informative" prior probability distribution, and tested its effectiveness using simulated data.

RESULTS

Our analyses showed that the mean life-span (≥ age 15) and the proportion of living persons aging over 45 were well-reproduced by the Gompertz model. The simulation showed that using the correlation as an informative prior provides a narrower estimation range in the Bayesian approach than does the uninformative prior.

CONCLUSIONS

The Gompertz model can be assumed to accurately estimate the mean life-span and/or the proportion of old people in a population. We suggest that the Strehler-Mildvan correlation can be used as a useful constraint in demographic reconstructions of past human populations.

Transition analysis: a validation study with known-age modern American skeletons

Transition Analysis-a recent skeletal age-estimation procedure (Boldsen et al.: Paleodemography: age distributions from skeletal samples (2002) 73-106)-is evaluated using 252 known-age modern American males and females from the Bass Donated Collection and Mercyhurst forensic cases. The pubic symphysis worked best for estimating age, followed by the sacroiliac joint and cranial sutures. Estimates based on all skeletal characteristics are influenced by the choice of prior distribution, although its effect is dwarfed by both the inaccuracy and imprecision of age estimates. Age intervals are narrowest for young adults, but are surprisingly short in old age as well. When using an informative prior distribution, the greatest uncertainty occurs from the late 40s into the 70s. Transition Analysis estimates do not perform as well as experience-based assessments, indicating the existing procedure is too narrowly focused on commonly used pelvic and cranial structures.

Estimation of age at death from the pubic symphysis and the auricular surface of the ilium using a smoothing procedure

We discuss here the estimation of age at death from two indicators (pubic symphysis and the sacro-pelvic surface of the ilium) based on four different osteological series from Portugal, Great-Britain, South Africa or USA (European origin). These samples and the scoring system of the two indicators were used by Schmitt et al. (2002), applying the methodology proposed by Lucy et al. (1996). In the present work, the same data was processed using a modification of the empirical method proposed by Lucy et al. (2002). The various probability distributions are estimated from training data by using kernel density procedures and Jackknife methodology. Bayes's theorem is then used to produce the posterior distribution from which point and interval estimates may be made. This statistical approach reduces the bias of the estimates to less than 70% of what was obtained by the initial method. This reduction going up to 52% if knowledge of sex of the individual is available, and produces an age for all the individuals that improves age at death assessment.

Skeletal estimation and identification in American and East European populations

Forensic science is a fundamental transitional justice issue as it is imperative for providing physical evidence of crimes committed and a framework for interpreting evidence and prosecuting violations to International Humanitarian Law (IHL). The evaluation of evidence presented in IHL trials and the outcomes various rulings by such courts have in regard to the accuracy or validity of methods applied in future investigations is necessary to ensure scientific quality. Accounting for biological and statistical variation in the methods applied across populations and the ways in which such evidence is used in varying judicial systems is important because of the increasing amount of international forensic casework being done globally. Population variation or the perceived effect of such variation on the accuracy and reliability of methods is important as it may alter trial outcomes, and debates about the scientific basis for human variation are now making their way into international courtrooms. Anthropological data on population size (i.e., the minimum number of individuals in a grave), demographic structure (i.e., the age and sex distribution of victims), individual methods applied for identification, and general methods of excavation and trauma analysis have provided key evidence in cases of IHL. More generally, the question of population variation and the applicability of demographic methods for estimating individual and population variables is important for American and International casework in the face of regional population variation, immigrant populations, ethnic diversity, and secular changes. The reliability of various skeletal aging methods has been questioned in trials prosecuted by the International Criminal Tribunal for the Former Yugoslavia (ICTY) in The Prosecutor of the Tribunal against Radislav Krstic (Case No. IT-98-33, Trial Judgment) and again in the currently ongoing trial of The Prosecutor of the Tribunal against Zdravko Tolimir, Radivolje Miletic, Milan Gvero, Vinko Pandurevic, Ljubisa Beara, Vujadin Popovic, Drago Nikolic, Milorad Trbic, Ljubomir Borovcanin (IT-05-88-PT, Second Amended Indictment). Following the trial of General Krstic, a collaborative research project was developed between the Forensic Anthropology Center at The University of Tennessee (UT) and the United Nations, International Criminal Tribunal for the Former Yugoslavia, Office of the Prosecutor (ICTY). The purpose of that collaboration was to investigate methods used for the demographic analysis of forensic evidence and where appropriate to recalibrate methods for individual estimation of age, sex, and stature for specific use in the regions of the former Yugoslavia. The question of "local standards" and challenges to the reliability of current anthropological methods for biological profiling in international trials of IHL, as well as the performance of such methods to meet the evidentiary standards used by international tribunals is investigated. Anthropological methods for estimating demographic parameters are reviewed. An overview of the ICTY-UT collaboration for research aimed at addressing specific legal issues is discussed and sample reliability for Balkan aging research is tested. The methods currently used throughout the Balkans are discussed and estimated demographic parameters obtained through medico-legal death investigations are compared with identified cases. Based on this investigation, recommendations for improving international protocols for evidence collection, presentation, and research are outlined.

Selectivity of black death mortality with respect to preexisting health

Was the mortality associated with the deadliest known epidemic in human history, the Black Death of 1347-1351, selective with respect to preexisting health conditions ("frailty")? Many researchers have assumed that the Black Death was so virulent, and the European population so immunologically naïve, that the epidemic killed indiscriminately, irrespective of age, sex, or frailty. If this were true, Black Death cemeteries would provide unbiased cross-sections of demographic and epidemiological conditions in 14th-century Europe. Using skeletal remains from medieval England and Denmark, new methods of paleodemographic age estimation, and a recent multistate model of selective mortality, we test the assumption that the mid-14th-century Black Death killed indiscriminately. Skeletons from the East Smithfield Black Death cemetery in London are compared with normal, nonepidemic cemetery samples from two medieval Danish towns (Viborg and Odense). The results suggest that the Black Death did not kill indiscriminately-that it was, in fact, selective with respect to frailty, although probably not as strongly selective as normal mortality.

Reconstruction of paleodemographic characteristics from skeletal age at death distributions: Perspectives from Hitotsubashi, Japan

This is a demographic exploration of the city of Edo, which reveals the changes that accompanied its urbanization and analyzes the skeletal remains of 207 individuals from a specific site in Tokyo (Hitotsubashi), using several paleodemographic approaches. A comparison of the three methods employed herein suggests that the Bayesian and maximum likelihood estimation techniques provide more plausible mortality patterns than the direct method of age estimation because the direct method of age estimation relies on published age intervals for the auricular surface and that would account for the underestimation of old people relative to the other two methods. Analyses using these new approaches indicate a short life span tendency for the people of Hitotsubashi. Although we cannot rule out methodological problems of adult-age estimation, one plausible interpretation of that life expectancy is an inadequate food supply and a poor public health situation. This study suggests that, in Tokugawa Japan, urbanization might have imposed health risks, increasing the risk of mortality. Analysis of demographic data from Hitotsubashi has refined our understanding on the impact of urbanization on the Edo period, and presents new perspectives on paleodemography in Japan.

Paleodemography of a medieval population in Japan: analysis of human skeletal remains from the Yuigahama-minami site

The purpose of this study is to obtain demographic data regarding the medieval population buried at the Yuigahama-minami site in Kamakura, Japan, and to detect a secular trend in the life expectancy of Japanese population over the last several thousand years. The Yuigahama-minami skeletal sample consists of 260 individuals, including 98 subadults (under 20 years old) and 162 adults. A Yuigahama-minami abridged life-table analysis yielded a life expectancy at birth (e0) of 24.0 years for both sexes, a life expectancy at age 15 years (e15) of 15.8 years for males, and an e15 of 18.0 years for females. The reliability of the estimated e0 was confirmed by analysis of the juvenility index. Demographic profiles comparing the Yuigahama-minami series with other skeletal series indicated that both the survivorship curve and life expectancy of the Yuigahama-minami sample are similar to those of the Mesolithic-Neolithic Jomon population, but are far lower than those of the early modern Edo population. These comparisons strongly suggest that life expectancy changed little over the thousands of years between the Mesolithic-Neolithic Jomon and medieval periods, but then improved remarkably during the few hundred years between the medieval period and early modern Edo period. The short-lived tendency of the Yuigahama-minami sample does not contradict the archaeological hypothesis of unsanitary living conditions in medieval Kamakura. This is the first investigation to address the demographic features of a medieval population in Japan, and will help refine our understanding of long-term trends in the demographic profiles of inhabitants of Japan.

Demographic trends and biological status of historic populations from Central Poland: the Ostrów Lednicki microregion

The microregion of Ostrów Lednicki in the province of Wielkopolska was the center of the formation of the Polish State in Early Middle Ages. The analysis of skeletal remains and parish records from the region documented the biological status of inhabitants and its changes over a millennium. The study material comprised 424 human skeletons from an Early Medieval burial ground on Lake Lednica, records of 2,704 deaths from the registers of the Roman Catholic parish of Dziekanowice, made between 1818-1903, data on the deaths of 929,192 inhabitants of rural areas all over the province Wielkopolska obtained from Prussian statistical materials for the years 1865-1900, and comparative data from the literature. Assuming both a stationary population model and a stable population model with nonzero natural increase, parameters of life tables and measures of opportunity for natural selection (Crow's index I(m), potential gross reproductive rate R(pot), and the biological state index I(bs)) were calculated for the Early Middle Ages and for the two periods of the 19th century which were characterized by different laws of land ownership and thus different rural economies. In the first period, peasants were tenants, whereas in the second, they were given freehold of the land they cultivated. Causes of death were also analyzed. A distinct increase in longevity from the early Middle Ages to the end of the 19th and the beginning of the 20th century was found. This was related to a higher level of demographic development in the parish of Dziekanowice during the 19th century, which was achieved earlier than in other areas of Poland. This was confirmed by genetic measures: coefficients of exogamy and coefficients of kinship. The reasons were related to the historical prominence of this region and to its proximity to the first two capitals of the Polish state, Gniezno and Poznań.

Differential skeletal preservation at Windover Pond: causes and consequences

In this paper, we evaluate the causes of differential skeletal preservation in the Windover Pond skeletal series (8BR246). We collected data on sex and age for approximately 110 individuals, and calculated a preservation score for each individual based on the presence of 80 skeletal landmarks. Our research questions evaluated the relationship between bone preservation and individual age and sex, and between the presence of preserved brain material and skeletal preservation, and the effects of burial location on bone preservation. The results indicate variability in average preservation for the sample (micro = 0.53, SD = 0.22) with an apparent lack of sex-specific (P = 0.79) or age-specific (P = 0.37) differences in preservation. The relationship between brain and skeletal preservation (P = 0.15) was not significant. The horizontal distribution of burials was not significantly correlated with skeletal preservation (north: r = -0.10, P = 0.93; east: r = 0.09, P = 0.45); however, vertical depth was a significant predictor of preservation (r = -0.31, P = 0.005), indicating that skeletal preservation decreased as burials were located closer to the ground surface. The observed variability in preservation scores may be related to the partial drying and resubmergence of the uppermost burials for the last few millennia. Comparison of Windover element-specific survival rates with previous analyses based on terrestrial samples (Galloway et al. [1997] Forensic taphonomy, Boca Raton: CRC Press; Waldron [1987] Death, decay and reconstruction, Manchester: Manchester University Press; Willey et al. [1997] Am J Phys Anthropol 104:513-528) affirms the relationship between element weight or density and bone survival. The unique taphonomic context of our study sample effected little change in bone deterioration processes.

Palaeodemography of the Nasca valley: reconstruction of the human ecology in the southern Peruvian coast

This study is based on skeletons and mummies belonging to 582 individuals excavated at sites of Pueblo Viejo, Cahuachi, Estaqueria and Atarco in the Nasca valley, South Coast of Peru. Archaeological evidence distinguishes three cultural phases: Nasca (400 BC-550 AD), Wari (600-1100 AD) and Chincha (1100-1412 AD). Since the Chincha human remains were too exiguous (27 individuals), only Nasca and Wari were considered. For the Nasca population, sex ratio was 113 men to 100 women (53% of males); for the Wari population, sex ratio was 117 men to 100 women (54% of males). The palaeodemographic data show that the infant mortality rate was 33@1000 for Nasca and 105@1000 for Wari. Life expectancy was 38-43 years for Nasca and 31-36 years for Wari. Death percentages in all the age groups increased from Nasca to Wari phase. ANOVA and t-test for paired comparison were applied in order to examine if dental and bone ages were statistically different. Long bones and teeth showed an allometric development, and the age estimated from the tooth formation and eruption was generally higher than the age estimated from the maximum lengths of long bones. The anthropological study of the Nasca valley skeletal populations confirmed the archaeological hypothesis of worse conditions of the Wari population in comparison with the previous Nasca people.

Deconstructing death in paleodemography

In 1992 in this Journal (Konigsberg and Frankenberg [1992] Am. J. Phys. Anthropol. 89:235-256), we wrote about the use of maximum likelihood methods for the "estimation of age structure in anthropological demography." More specifically, we presented a particular method (the "iterated age-length key") from the fisheries literature and suggested that the method could be used in human and primate demography and paleodemography as well. In our paper (section titled "Some Future Directions"), we spelled out two broad areas that we expected to see develop over the ensuing years. First, we felt that the use of explicit likelihood methods would open up interest in basic estimation issues, such as the calculation of standard errors for demographic estimates and the formulation of tests for whether samples differed in their demographic structure. Second, we felt that the time was ripe for hazards analyses that would incorporate the uncertainty in estimation that follows from using age "indicators" rather than known ages. While some of these developments have occurred during the last decade, few have been reported in the American Journal of Physical Anthropology. In this paper we resolve some issues from our 1992 paper, and attempt to redress this deficit in the literature by reviewing some recent developments in paleodemography over the past decade.

Semiparametric method for estimating paleodemographic profiles from age indicator data

This paper addresses the problem of estimating an age-at-death distribution or paleodemographic profile from osteological data. It is demonstrated that the classical two-stage procedure whereby one first constructs estimates of age-at-death of individual skeletons and then uses these age estimates to obtain a paleodemographic profile is not a correct approach. This is a consequence of Bayes' theorem. Instead, we demonstrate a valid approach that proceeds from the opposite starting point: given skeletal age-at-death, one first estimates the probability of assigning the skeleton into a specific osteological age-indicator stage. We show that this leads to a statistically valid method for obtaining a paleodemographic profile, and moreover, that valid individual age estimation itself requires a demographic profile and therefore is done subsequent to its construction. Individual age estimation thus becomes the last rather than the first step in the estimation procedure. A central concept of our statistical approach is that of a weight function. A weight function is associated with each osteological age-indicator stage or category, and provides the probability that a specific age indicator stage is observed, given age-at-death of the individual. We recommend that weight functions be estimated nonparametrically from a reference data set. In their entirety, the weight functions characterize the relevant stochastic properties of a chosen age indicator. For actual estimation of the paleodemographic profile, a parametric age distribution in the target sample is assumed. The maximum likelihood method is used to identify the unknown parameters of this distribution. As some components are estimated nonparametrically, one then has a semiparametric model. We show how to obtain valid estimates of individual age-at-death, confidence regions, and goodness-of-fit tests. The methods are illustrated with both real and simulated data.

Looking into the demography of an iron age population in the western Mediterranean. I. Mortality

In this paper, we attempt to reconstruct the mortality pattern of the population buried in S'Illot des Porros (Majorca), an Iron Age necropolis in the western Mediterranean, by means of paleodemographic analysis. The skeletal sample consists of 285 individuals, 93 subadults (under 20 years old) and 192 adults. The aim of this study is twofold: first, to identify and to evaluate the structural anomalies of the skeletal sample, and second, to obtain a possible and realistic description of the biological dynamics of this population, with special reference to its mortality pattern. The study uses current demographic methodology and several demographic models (for comparison). An abridged life table was built to estimate the mortality parameters. To evaluate the likelihood of the estimated data, an indirect analysis, which consisted of a comparison of our results with different population models (Weiss [1973] American Antiquity 38; Coale and Demeny [1996] Regional Model Life Tables and Stable Populations. Princeton: Princeton University Press; Ledermann [1969] Nouvelles tables-types de mortalité. Paris: Presses Universitaires de France), was carried out. An important bias was identified in the case of children, mostly affecting infants but also children between the ages of 1 and 5. This was interpreted as a census error due to taphonomic reasons and to an excluding differential funeral rite. A life expectancy at birth of approximately 28 years was estimated from the observed data. When this bias was removed, the estimated life expectancy at birth dropped to 23 years. The use of the Brass logit system allowed us to sketch a possible mortality profile for this population: low life expectancy, high infant mortality and hard life conditions, which were the cause of the low levels of survivorship in old ages. Am J Phys Anthropol 110:285-301, 1999.

RECENT ADVANCES IN METHOD AND THEORY IN PALEODEMOGRAPHY

▪ Abstract  Current methods in skeletal biology have improved significantly our ability to estimate the demographic parameters of extinct populations. Gross morphological and histological age indicators have been developed and tested in a variety of contexts, revealing great variation in the levels of accuracy of age prediction of each indicator. Primary attention is given here to the best-performing hard-tissue indicators of age and to composite methods of recovering the age and sex distribution of a cemetery. It is becoming increasingly apparent that some cemetaries should not be used for demographic reconstruction. Such collections have no bearing on the feasibility of paleodemographic research. Our review concludes with discussions about the role of comparing modern mortality patterns to those of paleodemography, and the issue and impact of departures from stationary demographic conditions during prehistoric times.

Use of ordinal categorical variables in skeletal assessment of sex from the cranium

In anthropological studies, visual indicators of sex are traditionally scored on an ordinal categorical scale. Logistic and probit regression models are commonly used statistical tools for the analysis of ordinal categorical data. These models provide unbiased estimates of the posterior probabilities of sex conditional on observed indicators, but they do so only under certain conditions. We suggest a more general method for sexing using a multivariate cumulative probit model and examine both single indicator and multivariate indicator models on a sample of 138 crania from a Late Mississippian site in middle Tennessee. The crania were scored for five common sex indicators: superciliary arch form, chin form, size of mastoid process, shape of the supraorbital margin, and nuchal cresting. Independent assessment of sex for each individual is based on pubic indicators. The traditional logistic regressions are cumbersome because of limitations imposed by missing data. The logistic regression correctly classified 66/74 males and 46/64 females, with an overall correct classification of 81%. The cumulative probit model classified 64/74 males correctly and 51/64 females correctly for an overall correct classification rate of 83%. Finally, we apply parameters estimated from the logit and probit models to find posterior probabilities of sex assignment for 296 additional crania for which pubic indicators were absent or ambiguous.

Estimation of African ape body length from femur length

The estimation from long bone lengths of stature in humans or body size in apes has a deep history in physical anthropology. To date, we can enumerate at least five different statistical methods for making such estimations. These methods are: (1) the regression of body length on long bone length (inverse calibration), (2) regression of long bone length on body length followed by solving for body length (classical calibration), (3) major axis regression of body length on long bone length, (4) reduced major axis regression of body length on long bone length, and (5) use of a long bone/body length ratio. We examine some of the statistical properties of these estimators using a large sample of humans (n = 2053) to derive the estimators, and applying them to smaller samples of Pan troglodytes (n = 42), Pan paniscus (n = 8), and Gorilla gorilla (n = 35). Based on the root mean-squared error (RMSE), the reduced major axis is the preferred estimator for body length in the combined Pan sample. However, inverse calibration is the best estimator for body length in gorillas based on the RMSE. Many estimators grossly underestimate body length in the apes. Differences in allometries between humans and great apes are obvious, but it is important to show the assumptions necessary in estimating body size from fossil remains, especially when isolated long bones are recovered and the global allometry is consequently unknown.

Effect of sample bias on paleodemographic fertility estimates

Paleodemographers must work to understand how representative any archaeologically recovered skeletal series is and the potential effects of series bias on their demographic reconstructions. We examine two forms of bias: 1) infant underenumeration caused by differential preservation or incomplete archaeological recovery and 2) the underenumeration of individuals over age 45 related to methodological bias. We generated 60 simulated skeletal series of 250 individuals each based on the Brass ([1971] Biological Aspects of Demography (London: Taylor and Francis), pp. 69-110) logit models. In the first test, age bias was introduced deterministically for all individuals with age at death over 40 years using the Lovejoy et al. ([1985] Am. J. Phys. Anthropol. 68:1-14) bias estimates. In the second test, 50% of all individuals under 5 years old were removed from each simulated distribution. The simulated series were analyzed using the model life table fitting procedure developed by the authors (Milner et al. [1989] Am. J. Phys. Anthropol. 80:49-58; Paine [1989] Am. J. Phys. Anthropol. 79:51-62). Forms of adult age estimation bias described by Lovejoy and coworkers inflate estimates by 10-20% of the true crude birth rate (CBR) (the number of births per year per 1,000 population). Overestimation of fertility and birth rates increases both absolutely and as a percentage of the true rate as population growth increases. This bias is very consistent. Because Lovejoy and colleagues have estimated the methodological bias itself, its effects can be estimated. Infant underenumeration is a more serious obstacle. It is not presently possible to estimate infant underenumeration reliably without prior knowledge of fertility rates. This reduces fertility reconstructions based on infant-biased samples to minimum fertility estimates.

Technical note: regression analysis in adult age estimation

Accurate estimation of human adult age has always been a problem for anthropologists, archaeologists and forensic scientists. The main factor contributing to the difficulties is the high variability of physiological age indicators. However, confounding this variability in many age estimation applications is a systematic tendency for age estimates, regardless of physiological indicator employed, to assign ages which are too high for young individuals, and too low for older individuals. This paper shows that at least part of this error is the inevitable consequence of the statistical procedures used to extract an estimate of age from age indicators, and that the magnitude of the error is inversely related to how well an age indicator is correlated with age. The use of classical calibration over inverse calibration is recommended for age estimation.

Assessing the reliability of paleodemographic fertility estimators using simulated skeletal distributions

The reliability of published paleodemographic fertility reconstruction methods was assessed using simulated age-at-death distributions and a published cemetery series from a population with known birth rates. In the first test, the Brass ([1971] Biological Aspects of Demography, pp. 69-110) LOGIT models were used to generate 180 simulated skeletal samples of various sizes (N = 50, 100, 250) from hypothetical populations with known demographic rates. The base populations were expanding (r = 0.01), stationary, or declining (r = -0.01), yet all had the same life expectancy. Growth differences resulted from different fertility rates. The simulated skeletal series were then analyzed using the model life table fitting procedure outlined by Paine ([1989a] Am. J. Phys. Anthropol. 79:51-62), three commonly employed age ratio tests (Bocquet-Appel and Masset [1892] J. Hum. Evol. 11:321-333; Buikstra et al. [1986] Am. Antiquity 51:528-546), and one age-at-death ratio not previously published. In the second test the model life table fitting procedure was used to estimate fertility for a historical population, the Newton Plantation, Barbados (Corruccini et al. [1989] Am. Antiquity 54:609-614), with known demographic characteristics.