The history of Central African Pygmy and Bantu-speaking farmer populations (2/9/2008)

Researchers from CNRS and Institut Pasteur(1), working with an interdisciplinary and international team, have studied the demographic and genetic history of Central African Pygmee and Bantus-speaking farmer populations. The study suggests that the two groups started to diverge from a common ancestral population some 70,000 years ago. The groups then remained isolated from one another before renewed genetic exchange starting 40,000 years ago, through marriage between Pygmy women and male farmers. Once these data are confirmed by other independent genetic markers, the results will serve as a base for studying the impact of sedentarization on the evolution of the genome and in particular on vulnerability or resistance to pathogens.

Population geneticists from CNRS and Institut Pasteur worked with researchers in bioinformatics, ethnolinguistics and epidemiology(2) to study Pygmy (nomadic hunter-gatherers) and Bantu-speaking villager (farmers and sedentary herders) populations in Central Africa. The goal was to determine to what extent social, cultural and demographic factors have influenced the genetic heritage of these populations.

The researchers used mitochondial DNA(3) (ADNmt), which is transmitted only through female lineage. Their population sample was made up of 1500 individuals from 20 Bantu-speaking farmer populations and 9 hunter-gatherer Pygmy populations from Gabon, Cameroon, the Central African Republic, and the Democratic Republic of Congo.

The researchers identified an ancestral and autochthonous line of ADNmt from Central Africa, which was formerly shared by Western Pygmees and farmers. This line evolved into a single lineage among modern Western Pygmees and a great diversity of lineages among the farmers. Generally speaking, variability in ADNmt among Pygmees is much weaker than among farmers: the maternal gene pool among modern Pygmees comes from a small number of common ancestors.

This study suggests that Pygmees started to diverge from the ancestral population at most 70,000 years ago. After a period of isolation, during which current phenotype differences between Pygmees and farmers built up, Pygmy women started marrying male farmers (but not the opposite). This trend started at most 40,000 years ago, and continued until at least several thousand years ago. Subsequently, the gene pool of the Pygmees was not enriched by external gene influxes. The farmer gene pool, on the other hand, was enriched during the so-called "Bantu expansions", an event corresponding to technological, demographic and linguistic advances in the late Stone Age.

The researchers now plan to study nuclear DNA, notably the Y chromosome, to verify their conclusions. They chose Western Africa because it is one of the few regions where nomadic and sedentary populations cohabit. They eventually want to study the relationships between the genome and the populations' vulnerability or resistance to pathogens. The transition into sedentary lifestyle is often accompanied by three factors which have an impact on pathogens: demographic growth, which allows the pathogens to spread more easily; the presence of refuse where people live, which is a vector for illness; and the presence of farm animals, which have illnesses which are more likely to jump to man.

This research was funded by a number of programs: the « Origine de l'homme, des langues et du langage » program (CNRS), the « Origin of Man, Languages and Language» EUROCORES from the European Science Foundation and the « Histoire et diversité des Pygmées de l'Afrique Centrale et de leurs voisins » ACI Prosodie program from the French Ministry of Research.

Notes

1. Laboratoire Hôtes, vecteurs et agents infectieux : biologie et dynamique, Equipe « Génétique Evolutive Humaine » (CNRS/Institut Pasteur)
2. Laboratoire Eco-anthropologie et ethnobiologie (CNRS/Muséum d'histoire naturelle/Université Paris) et Dynamique du langage (CNRS/Université Lyon 2), en collaboration avec les Universités de Barcelone, de Haifa, de St Jacques de Compostelle et de Yale, le Centre d'Etudes du Polymorphisme Humain (CEPH-Fondation Jean Dausset) à Paris et le CIRMF de Franceville.
3. Les mitochondries sont des organites cellulaires qui permettent aux cellules de respirer. Elles possèdent leur propre ADN, dit ADN mitochondrial.

Note: This story has been adapted from a news release issued by CNRS

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