Reconstructing Prehistoric Demography: What Role for Extant Hunter-Gatherers?
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Received: 3 October 2019 Revised: 17 February 2020 Accepted: 6 October 2020 DOI: 10.1002/evan.21869 REVIEW ARTICLE Reconstructing prehistoric demography: What role for extant hunter-gatherers? Abigail E. Page1 | Jennifer C. French2 1Department of Population Health, London School of Hygiene and Tropical Medicine, Abstract London, UK Demography is central to biological, behavioral, and cultural evolution. Knowledge of 2 Department of Archaeology, Classics and the demography of prehistoric populations of both Homo sapiens and earlier mem- Egyptology, University of Liverpool, Liverpool, UK bers of the genus Homo is, therefore, key to the study of human evolution. Unfortu- nately, demographic processes (fertility, mortality, migration) leave little mark on the Correspondence Abigail E. Page, Department of Population archeological and paleoanthropological records. One common solution to this issue is Health, London School of Hygiene and the application of demographic data from extant hunter-gatherers to prehistory. Tropical Medicine, London, UK. Email: [email protected] With the aim of strengthening this line of enquiry, here we outline some pitfalls and their interpretative implications. In doing so, we provide recommendations about the Funding information Leverhulme Trust, Grant/Award Number: ECF- application of hunter-gatherer data to the study of demographic trends throughout 2016-128; Medical Research Council, Grant/ human evolution. We use published demographic data from extant hunter-gatherers Award Number: MR/P014216/1; Wenner- Gren Foundation, Grant/Award Number: 9862 to show that it is the diversity seen among extant hunter-gatherers—both intra- and inter-population variability—that is most relevant and useful for understanding past hunter-gatherer demography. KEYWORDS demography, fertility, hunter-gatherers, life history, prehistory 1 | INTRODUCTION Our knowledge of demography in prehistory, however, faces one fundamental obstacle: the lack of direct data. Fertility, mortality, and Demography and evolution are intrinsically intertwined. Evolution is migration leave little mark on the archeological and paleoanthropolog- driven by the propagation of genes, which is determined by the key ical records. Researchers studying the outcome of variation in these processes of survival (mortality), fertility, and dispersal (migration) of demographic processes (changes in population size, density, and individuals.1,2 Demographic trends influence the age- and sex- growth rate) have met with more success, but are largely limited to structure of a society, altering the balance between kin and non-kin the comparison of relative changes in these measures.9 Furthermore, within individuals' social networks,3 with consequences for coopera- most established paleodemographic methods10–14 have limited appli- tion, parental investment, and information exchange.4–6 Demography cability to the Pleistocene contexts (2.6 million to 12,000 years further influences the development and variability of human culture, ago15) that constitute much of prehistory. Genetic data (from both through its recognized role in trait transmission and cumulative cul- ancient and living populations) inform on some elements of prehistoric ture.7,8 Knowledge of the demography of prehistoric populations of demography, notably interbreeding between different hominin taxa, both Homo sapiens and earlier members of the genus Homo thus has and estimates of effective population sizes,16,17 but address a limited the potential to offer vital insights into human evolution across multi- range of demographic questions, and ancient DNA sequences are ple domains. comparatively rare. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. © 2020 The Authors. Evolutionary Anthropology published by Wiley Periodicals LLC. Evolutionary Anthropology. 2020;1–14. wileyonlinelibrary.com/journal/evan 1 2 PAGE AND FRENCH A common solution to this lack of direct data is to look to the deaths, and migration, fieldworkers conduct detailed self-reported richer corpus of demographic data on extant hunter-gatherers. Demo- genealogies. However, self-reports rarely produce a fully accurate graphic data from recent ethnographically-documented hunter- demographic record; recall bias is common, leading to an gatherer populations are important tools in prehistoric demographic underreporting of births, miscarriages, stillbirths, and infant mortality research. Ethnographic data play a key role in developing frameworks due to either forgetfulness, a lack of cultural recognition of a “birth” of prehistoric demography (e.g., Reference 19) and providing absolute or simple miscommunication.6,25 Taboos may further exacerbate inac- values of prehistoric demographic variables (population size, density, curacies. For example, the Agta use nicknames to refer to their in-laws and fertility and mortality rates) for use as descriptive statistics and/or since it is forbidden to use their names. As different nicknames are input parameters in formal demographic models.20–24 While certainly used, reconciling different genealogies can be challenging.26 Self- not complete, nor without its own issues (something we return to reporting is also an issue for aging; hunter-gatherers often have a very below), compared to the archeological and paleoanthropological different, or no concept of calendar time, thus, no clear idea of how records, a mass of quantitative data on fertility, and to a lesser degree, old they are.27 Precise estimates of ages are fundamental to the con- mortality, and migration, are available for multiple hunter-gatherer struction of an accurate demographic record, and the subsequent populations. Here, we examine critically the wider relevance and study of life history. Obtaining accurate age estimates is the major application of demographic data from extant hunter-gatherers to pre- challenge of hunter-gatherer demographic studies (Box 2). history (with a particular focus on the Pleistocene). The explicit aim of Researchers face two further key issues. First, hunter-gatherer this paper is to highlight key theoretical and methodological consider- population sizes are small (800–1,000 individuals28,29) and are espe- ations in both producing and using hunter-gatherer demographic data cially vulnerable to stochastic (random) demographic variation. In to study prehistoric demography. Consequently, our main goal is to terms of long-term growth or decline, in a large population, individual communicate these considerations beyond evolutionary anthropol- events are “averaged” out, while in smaller groups this random varia- ogy, and address our colleagues in archeology and paleoanthropology, tion has disproportionate influence; patterns of growth and decline who may be unfamiliar with the relevant literature and prevailing are far more extreme and volatile in small populations.30,31 Conse- approaches. quently, in a small population an extremely low or high fertility rate We highlight five pitfalls faced by researchers working with past may be the outcome of the size of the sample. HBE frequently explore and present hunter-gatherers, and thus applicable to both anthropolo- whether a behavior is adaptive32 and a surprisingly low fertility rate gists and archeologists. These pitfalls are not new, and we are not the can be interpreted as the product of maladaptive behavior. This may first to recognize them. However, our analysis suggests that these pit- be the case. However, we must also consider that a fertility rate is the falls are not clearly communicated beyond disciplinary confines (Box product of a random collection of individual events, reflective of the 1). Reasons for opaqueness are miscommunication of methodology, study population, and data collection protocols (i.e., few births occur assumptions which conflict with wider biological principles, and the in the population during data collection). Furthermore, small sample uncritical application of single group demographic values or “hunter- sizes are one reason why data on older age mortality is lacking in com- gatherer” averages to the past. This has important implications for our parison to data on fertility rates and pre-adult (age 16 and below) understanding of prehistoric demography. We draw on data from mortality in hunter-gatherers. As the cohort ages more individuals die, extant hunter-gatherers to argue that the between and within- and the sample at risk of death becomes smaller. With small population variability in demographic variables seen among extant populations, the sample size at older ages is likely too small to accu- hunter-gatherers is best equipped to provide insights into past rately estimate mortality rates (i.e., when the mortality rate is less than hunter-gatherer demography. Through the lens of human behavioral 1/n18,33). Given the importance of age-specific mortality rates in ecology (HBE) these data allow us to examine how hunter-gatherers human evolution, demography and life history models33 (see pitfall today react to selective pressures, providing the tools with which to two) researchers working on prehistory should pay attention to the identify parallels in prehistory. sample sizes from which key figures arise, acknowledging the large confidence intervals around the mortality estimates. The relatively short timeframe of ethnographic fieldwork intro- 2 | PITFALL ONE: NOT RECOGNIZING THE