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Post marital residence behaviours shape genetic variation in hunter-gatherer and agricultural populations from Central Africa

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Abstract

The evolution of the sociocultural behaviours determining lineage transmission and post marital residence rules in human societies has been the subject of intense debates in cultural anthropology and ethnology. Population genetics paradigms and methods can provide new insights into the past mobility behaviour of spouse and their influence on the genetic diversity patterns observed today among communities and populations. Here, we review, for a non-specialized scientific audience, two population genetics papers that we previously published (Verdu et al 2009; 2013). These papers mainly focused on the reconstruction of the demographic history of Central African hunter-gatherer and agricultural populations using genetic data, and on how complex post marital residence behaviours shaped the genetic diversity patterns currently observed within and among these populations. In particular, we found that Central African hunter-gatherer populations show genetic patterns that would be expected for matrilocal populations, while they often report practicing patrilocality similarly as their neighbouring agricultural populations. In fact, we showed that complex and variable gender-specific post marital residence behaviours in the context of patrilocality and socioeconomic discrimination of hunter-gatherer populations could satisfactorily explain the observed genetic patterns. Nevertheless, as an alternative explanation, we propose here that these patterns may result also from past post marital residence rules that are not currently observed in these Central African hunter-gatherer and agricultural populations.

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... Among farming populations, genetic studies have evidenced high levels of genetic similarity in autosomal markers, and this genetic homogeneity is likely due to the recent demic expansion of Bantuspeaking populations concomitant with the expansion of agricultural techniques and Bantu languages starting about 4-5 kya [68][69][70][71][72]. However, if we focus on sex-specific genetic diversity patterns in these populations, genetic studies show more genetic differentiation among populations for genetic markers in the Y-chromosome than in the mtDNA [16,[68][69][70][71]. ...
... These studies provide strong evidence for a higher female than male migration rate among farming populations from sub-Saharan Africa. This is consistent with the self-reported patrilocal practices in farming populations, where females move after marriage across communities and populations much more often than males [69,72]. Sex-specific genetic patterns in farming populations may also be linked to the practice of polygyny, which reduces the genetic diversity of male lineages compared to female lineages [65]. ...
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Africa is the continent of our species’ origin and the deep history of humans is represented by African genetic variation. Through genetic studies, it has become evident that deep African population history is captured by relationships among African hunter-gatherers and that the world’s deepest population divergences occur among these groups. In this review, we look back at a study published by Cavalli-Sforza and colleagues in 1969 entitled “Studies on African Pygmies. I. A pilot investigation of Babinga Pygmies in the Central African Republic (with an analysis of genetic distances)”. The study analyzed 19 classical polymorphisms and found that the deepest divergences in African populations were represented by hunter-gatherer groups such as the southern African San and the central African rainforest hunter-gatherers. We repeated the original analyses from Cavalli-Sforza et al. [1] with about 22 thousand times more genome-wide genetic markers in populations similar to those included in the original study. Our high-resolution analyses gave similar results regarding the relationships of early-diverging African populations compared to the classical polymorphism analyses. This finding, however, does not imply that research has stagnated and that developments in technology and genetic methods over the last fifty years delivered no additional information regarding African history and adaptation. We review how technology and population genetic methods have advanced to give more detailed inferences about population structure, migrations, admixture patterns, timing of admixture, sex-biased admixture, and inferences of selection and adaptive introgression in rainforest hunter-gatherers and other African populations. We also comment on how sequencing of ancient DNA has influenced findings and deliberate on the progress and development of more complex models of African history, including alternatives to tree-models and the inference of possible archaic admixture in African populations. We review the growing complexity of our picture of population history in central Africa and Africa as a whole, emerging from genomic studies and other disciplines investigating human population history and adaptation. While data and knowledge are accumulating, certain populations and areas remain underrepresented in genomic research. Their inclusion, possibly also through ancient DNA studies, together with new methods of analysis and the testing of representative models of deep population history in Africa, will help to build a more complete picture of past population history in Africa. Based on “Cavalli-Sforza LL, Zonta LA, Nuzzo F, Bernini L, de Jong WW, Meera Khan P, Ray AK, Went LN, Siniscalco M, Nijenhuis LE, van Loghem E, Modiano G. Studies on African Pygmies. I. A pilot investigation of Babinga Pygmies in the Central African Republic (with an analysis of genetic distances). Am J Hum Genet. 1969 May;21(3):252-274”.
... Population genetics studies reveal a distinct matrilocal bias, evident in the localization of mother-todaughter mtDNA lineages compared to father-to-son Y-lineages. This is opposite to the pattern in neighboring farming and pastoralist populations (Schlebusch, 2010, for Khoesan;Verdu & Austerlitz, 2015, for Central African Pygmies). A similar tendency for female relatives to stay together is found in Hadza longitudinal residence data (Blurton Jones, Hawkes, & O'Connell, 2005;Wood & Marlowe, 2011;Woodburn, 1968). ...
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Fertility inheritance, a phenomenon in which an individual's number of offspring is positively correlated with his or her number of siblings, is a cultural process that can have a strong impact on genetic diversity. Until now, fertility inheritance has been detected primarily using genealogical databases. In this study, we develop a new method to infer fertility inheritance from genetic data in human populations. The method is based on the reconstruction of the gene genealogy of a sample of sequences from a given population and on the computation of the degree of imbalance in this genealogy. We show indeed that this level of imbalance increases with the level of fertility inheritance, and that other phenomena such as hidden population structure are unlikely to generate a signal of imbalance in the genealogy that would be confounded with fertility inheritance. By applying our method to mtDNA samples from 37 human populations, we show that matrilineal fertility inheritance is more frequent in hunter-gatherer populations than in food-producer populations. One possible explanation for this result is that in hunter-gatherer populations, individuals belonging to large kin networks may benefit from stronger social support and may be more likely to have a large number of offspring.
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Human genetic diversity is shaped by both demographic and biological factors and has fundamental implications for understanding the genetic basis of diseases. We studied 938 unrelated individuals from 51 populations of the Human Genome Diversity Panel at 650,000 common single-nucleotide polymorphism loci. Individual ancestry and population substructure were detectable with very high resolution. The relationship between haplotype heterozygosity and geography was consistent with the hypothesis of a serial founder effect with a single origin in sub-Saharan Africa. In addition, we observed a pattern of ancestral allele frequency distributions that reflects variation in population dynamics among geographic regions. This data set allows the most comprehensive characterization to date of human genetic variation.
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Opening Paragraph Un énorme travail a été réalisé sur les populations Pygmées du Zaïre. Dans ce domaine, l'ethnologie, l'anthropologie, la théologie africaine doivent beaucoup à J.-P. Hallet, à P. Schebesta, à C. M. Turnbull et á bien d'autres. Bon nombre de ces études ont diffusé et privilgié l'image prédominante du Pygmée nomadisant á travers la forêt, son milieu naturel. Cependant, elles ont occulté la connaissance et empêché l'investigation sur de nombreux autres groupes de Pygmées vivant loin de la forêt. En effet, pour Schebesta, qui fait de l'anthropométrie un critère privilégié, les seuls vrais Pygmées sont les Bambuti de la forêt de l'Ituri (1933: 63). Tous les autres ne seraient que des ‘Pygmoformes’ ou des ‘Pygmoïdes’, des ‘peuples métissés dont un croisement aura abâtardi les caractères de race et de culture’ (1940: 20). De nos jours, les critéres ont changé. Certains chercheurs se fondent sur le caractère polyphonique de la musique comme élément primordial dans la définition du Pygmée.
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Guided by life-history theory, we examine the contexts in which reproductive interests (e.g., attracting and keeping a spouse) shape forager mobility. Using quantitative intercultural and intracultural mating distance data, we report significant correlations between mating distance and population density not only among foragers but also among farmers. Further, in all of the cultures for which we could find data males on average traveled considerably farther than females in a lifetime, and among Aka foragers of Central Africa age and sex significantly impacted distances traveled. We propose a descriptive model of forager mobility that supplements existing subsistence-focused models by incorporating reproductive interests at all levels. © 1999 by The Wenner-Gren Foundation for Anthropological Research. All rights reserved.
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New linguistic evidence about the classification of the Bantu languages does not support the current view that these languages spread as the result of a massive migration or ‘expansion’ by its speakers. Rather the present geographic distribution of Bantu languages is the outcome of many complex historical dynamics involving successive dispersals of individual languages over a time span of millennia and involving reversals as well as successes. This is as true for eastern and southern Africa, where a close correlation between the archaeological evidence documenting the diffusion of basic food-related technologies, including metallurgy and the spreading of Bantu languages has become an axiom, as it is elsewhere. The linguistic evidence concerning the dispersal of Bantu languages in these regions of Africa is completely incongruent with the archaeological record. The existing Bantu expansion hypothesis must be totally abandoned. The scrapping of the hypothesis will make room for more realistic and quite different interpretations and research hypotheses. For example, it follows that the local or regional contribution of speakers of other languages, autochthons and others, to the development of later cultures and societies was probably considerably greater than has hitherto been acknowledged and that the continuities in historical dynamics of all sorts between the Bantu-speaking parts of Africa and areas further north and west are greater than has been hitherto realized.
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In this article I am illustrating the linguistic diversity of African Pygmy populations in order to better address their anthropological diversity and history. I am also introducing a new method, based on the analysis of specialized vocabulary, to reconstruct the substratum of some languages they speak. I show that Pygmy identity is not based on their languages, which have often been borrowed from neighboring non-Pygmy farmer communities with whom each Pygmy group is linked. Understanding the nature of this partnership, quite variable in history, is essential to addressing Pygmy languages, identity, and history. Finally, I show that only a multidisciplinary approach is likely to push forward the understanding of African Pygmy societies as genetic, archeological, anthropological, and ethnological evidence suggest.
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In the human species, the two uniparental genetic systems (mitochondrial DNA and Y chromosome) exhibit contrasting diversity patterns. It has been proposed that sex-specific behaviours, and in particular differences in migration rate between men and women, may explain these differences. The availability of high-density genomic data and the comparison of genetic patterns on autosomal and sex chromosomes at global and local scales allow a reassessment of the extent to which sex-specific behaviours shape our genome. In this article, we first review studies comparing the genetic patterns at uniparental and biparental genetic systems and assess the extent to which sex-specific migration processes explain the differences between these genetic systems. We show that differences between male and female migration rates matter, but that they are certainly not the only contributing factor. In particular, differences in effective population size between men and women are also likely to account for these differences. Then, we present and discuss three anthropological processes that may explain sex-specific differences in effective population size and thus human genomic variation: (i) variance in reproductive success arising from, for example, polygyny; (ii) descent rules; and (iii) transmission of reproductive success.
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The current distribution of Bantu languages is commonly considered to be a consequence of a relatively recent population expansion (3-5kya) in Central Western Africa. While there is a substantial consensus regarding the centre of origin of Bantu languages (the Benue River Valley, between South East Nigeria and Western Cameroon), the identification of the area from where the population expansion actually started, the relation between the processes leading to the spread of languages and peoples and the relevance of local migratory events remain controversial. In order to shed new light on these aspects, we studied Y chromosome variation in a broad dataset of populations encompassing Nigeria, Cameroon, Gabon and Congo. Our results evidence an evolutionary scenario which is more complex than had been previously thought, pointing to a marked differentiation of Cameroonian populations from the rest of the dataset. In fact, in contrast with the current view of Bantu speakers as a homogeneous group of populations, we observed an unexpectedly high level of interpopulation genetic heterogeneity and highlighted previously undetected diversity for lineages associated with the diffusion of Bantu languages (E1b1a (M2) sub-branches). We also detected substantial differences in local demographic histories, which concord with the hypotheses regarding an early diffusion of Bantu languages into the forest area and a subsequent demographic expansion and migration towards eastern and western Africa.
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The past decade of advances in molecular genetic technology has heralded a new era for all evolutionary studies, but especially the science of human evolution. Data on various kinds of DNA variation in human populations have rapidly accumulated. There is increasing recognition of the importance of this variation for medicine and developmental biology and for understanding the history of our species. Haploid markers from mitochondrial DNA and the Y chromosome have proven invaluable for generating a standard model for evolution of modern humans. Conclusions from earlier research on protein polymorphisms have been generally supported by more sophisticated DNA analysis. Co-evolution of genes with language and some slowly evolving cultural traits, together with the genetic evolution of commensals and parasites that have accompanied modern humans in their expansion from Africa to the other continents, supports and supplements the standard model of genetic evolution. The advances in our understanding of the evolutionary history of humans attests to the advantages of multidisciplinary research.
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In this paper, we present a study of genetic variation in sub-Saharan Africa, which is based on published and unpublished data on fast-evolving (hypervariable region 1 of mitochondrial DNA and six microsatellites of Y chromosome) and slow-evolving (haplogroup frequencies) polymorphisms of mtDNA and Y chromosome. Our study reveals a striking difference in the genetic structure of food-producer (Bantu and Sudanic speakers) and hunter-gatherer populations (Pygmies, Kung, and Hadza). In fact, the ratio of mtDNA to Y-chromosome Nupsilon is substantially higher in food producers than in hunter-gatherers as determined by fast-evolving polymorphisms (1.76 versus 0.11). This finding indicates that the two population groups differ substantially in female and male migration rate and/or effective size. The difference also persists when linguistically homogeneous populations are used and outlier populations are eliminated (1.78 vs 0.19) or when the jacknife procedure is applied to a paired population data set (1.32 to 7.84 versus 0.14 to 0.66). The higher ratio of mtDNA to Y-chromosome Nnu in food producers than in hunter-gatherers is further confirmed by the use of slow-evolving polymorphisms (1.59 to 7.91 versus 0.12 to 0.35). To explain these results, we propose a model that integrates demographic and genetic aspects and incorporates ethnographic knowledge. In such a model, the asymmetric gene flow, polyginy, and patrilocality play an important role in differentiating the genetic structure of sub-Saharan populations. The existence of an asymmetric gene flow is supported by the phylogeographic features of mtDNA and Y-chromosome haplogroups found in the two population groups. The role of polyginy and patrilocality is sustained by the evidence of a differential pressure of genetic drift and gene flow on maternal and paternal lineages of food producers and hunter-gatherers that is revealed through the analysis of mitochondrial and Y-chromosomal intrapopulational variation.
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