More specifically, palaeodemography looks at the changes in pre-modern populations in order to determine something about the influences on the lifespan and health of earlier peoples.[citation needed] Reconstructions of ancient population sizes and dynamics are based on bioarchaeology,[2]ancient DNA, and inference from modern population genetics.[citation needed]
Methods
Skeletal analysis
Skeletal analysis can yield information such as an estimation of age at time of death. There are numerous methods that can be used;[3] in addition to age estimation and sex estimation, someone versed in basic osteology can ascertain a minimum number of individuals (or MNI) in cluttered contexts—such as in mass graves or an ossuary. This is important, as it is not always obvious how many bodies compose the bones sitting in a heap as they are excavated.
Occasionally, historical disease prevalence for illnesses such as leprosy can also be determined from bone restructuring and deterioration. Paleopathology, as these investigations are called, can be useful in accurate estimation of mortality rates.
Genetic analysis
The increasing availability of DNA sequencing since the late 1990s has allowed estimates on Paleolithic effective population sizes.[4][5][6]
Such models suggest a human effective population size of the order of 10,000 individuals for the Late Pleistocene. This includes only the breeding population that produced descendants over the long term, and the actual population may have been substantially larger (in the six digits).[7]
Sherry et al. (1997) based on Alu elements estimated a roughly constant effective population size of the order of 18,000 individuals for the population of Homo ancestral to modern humans over the past one to two million years.[8] Huff et al. (2010) rejected all models with an ancient effective population size larger than 26,000.[9] For ca. 130,000 years ago, Sjödin et al. (2012) estimate an effective population size of the order of 10,000 to 30,000 individuals, and infer an actual "census population" of early Homo sapiens of roughly 100,000 to 300,000 individuals.[10]
The authors also note that their model disfavours the assumption of an early (pre-Out-of-Africa) population bottleneck affecting all of Homo sapiens.[11]
Estimates of habitable land area
According to a 2015 study, the total land area of Africa, Eurasia, and Sahul that was habitable to humans during the Last Glacial Maximum (LGM) was around 76,959,712.4 km2. Based on a dataset of average population density of hunter-gatherer groups collected by Lewis R. Binford, which indicate a mean density of 0.1223 humans per km2 and a median density of 0.0444 humans per km2, the combined human population of Africa and Eurasia at the time of the LGM would have been between 2,998,820 and 8,260,262 people. Alternatively, if a human population density based on that of modern medium to large-bodied carnivores, whose median density is 0.0275 individuals per km2 and whose mean density is 0.0384 individuals per km2, is used, a total Afro-Eurasian human population of 2,120,000 to 2,950,000 is obtained. Sahul's population density was significantly lower than that of Afro-Eurasia, being calculated as only 0.005 humans per km2 during the time just prior to the LGM. As a consequence, assuming Sahul possessed an estimated total habitable land area of 9,418,730.8 km2, its population was at most 47,000 at the time of the LGM, and probably less than that given that its population is believed to have declined by as much as 61% during the LGM,[12] a demographic trend supported by archaeological evidence,[13] and it thus would have possessed an even lower actual population density than the calculated density from just before the LGM.[14]
Hominid population estimates
It is estimated by J. Lawrence Angel [15] that the average life span of hominids on the Africansavanna between 4,000,000 and 200,000 years ago was 20 years. This means that the population would be completely renewed about five times per century,[citation needed] assuming that infant mortality has already been accounted for[clarification needed]. It is further estimated that the population of hominids in Africa fluctuated between 10,000 and 100,000 individuals, thus averaging about 50,000 individuals[clarification needed]. Multiplying 40,000 centuries by 50,000 to 500,000 individuals per century yields a total of 2 billion to 20 billion hominids that lived during that approximately 4,000,000-year time span.[16]
^Eller, Elise; Hawks, John; Relethford, John H. (2009). "Local Extinction and Recolonization, Species Effective Population Size, and Modern Human Origins". Human Biology. 81 (5–6): 805–24. doi:10.3378/027.081.0623. PMID20504198. S2CID27753579. The relationship between census size and effective size is complex, but arguments based on an island model of migration show that if the effective population size reflects the number of breeding individuals and the effects of population subdivision, then an effective population size of 10,000 is inconsistent with the census size of 500,000 to 1,000,000 that has been suggested by archeological evidence. However, these models have ignored the effects of population extinction and recolonization, which increase the expected variance among demes and reduce the inbreeding effective population size. Using models developed for population extinction and recolonization, we show that a large census size consistent with the multiregional model can be reconciled with an effective population size of 10,000, but genetic variation among demes must be high, reflecting low interdeme migration rates and a colonization process that involves a small number of colonists or kin-structured colonization.
^Sjödin, Per; Sjöstrand, Agnès E; Jakobsson, Mattias; Blum, Michael G. B. (2012). "Resequencing data provide no evidence for a human bottleneck in Africa during the penultimate glacial period". Mol Biol Evol. 29 (7): 1851–60. doi:10.1093/molbev/mss061. PMID22319141. A small human effective population size, on the order of 10,000 individuals, which is smaller than the effective population size of most great apes, has been interpreted as a result of a very long history, starting ? 2 mya, of a small population size, coined as the long-necked bottle model (Harpending et al. 1998; Hawks et al. 2000). Our findings are consistent with this hypothesis, but, depending on the mutation rate, we find either an effective population size of NA = 12,000 (95% C.I. = 9,000–15,500 when averaging over all three demographic models) using the mutation rate calibrated with the human-chimp divergence or an effective population size of NA = 32,500 individuals (95% C.I. = 27,500–34,500) using the mutation rate given by whole-genome trio analysis (The 1000 Genomes Project Consortium 2010) (supplementary figure 4 and table 6, Supplementary Material online). Not surprisingly, the estimated effective mutation rates ? = 4NAµ are comparable for the two mutation rates we considered, and are equal to 1.4 × 10?3/bp/generation (95% C.I. = (1.1–1.7) × 10?3). Relating the estimated effective population size to the census population size during the Pleistocene is a difficult task because there are many factors affecting the effective population size (Charlesworth 2009). Nevertheless, based on published estimates of the ratio between effective and census population size, a comprehensive value on the order of 10% has been found by Frankham (1995). This 10% rule roughly predicts that 120,000–325,[0]00 individuals (depending on the assumed mutation rate)
^Sjödin et al. 2012, In contrast to the bottleneck theory, we show that a simple model without any bottleneck during the penultimate ice age has the greatest statistical support compared to bottleneck models.
^Angel, J. Lawrence (May 1969). "The bases of paleodemography". American Journal of Physical Anthropology. 30 (3): 427–437. doi:10.1002/ajpa.1330300314. PMID5791021 – via doi:10.1002/ajpa.1330300314. PMID 5791021.
Hoppa, Robert D., Vaupel, James W., Paleodemography: Age Distributions from Skeletal Samples, 2008, Cambridge University Press, ISBN1139441558, 9781139441551, google books
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