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Population Genetics

Common Origins of Pygmies and Bantus

A team from CNRS and the Institut Pasteur (1), working with researchers in bioinformatics, ethnolinguistics, and epidemiology, (2) suggests that Central African Pygmies and Bantus, two physically very different ethnic groups, branched out from a common ancestral population some 70,000 years ago. (3)


pygmies

© S. Bahuchet/MNHN

Aka pygmies are hunter-gatherers : exchange of goods with a Bantu woman.



The term “Bantu” applies to all African populations in whose language the word “bantu” means man. In the 1960s, it became a general label for over 400 ethnic groups spread all over sub-Saharan Africa. Bantus are mainly rural farmers and sedentary herders. Pygmies are hunters-gatherers living in small seminomadic bands in the rainforest, and they are generally much shorter than their Bantu neighbors. The average height of adult male Pygmies is less than 1m50 (5 feet).
But scientists will not stop at such an obvious difference. A CNRS-Pasteur team studied the difference between Bantu and Pygmy populations by looking at differences in mitochondrial DNA (mtDNA)4 in the two groups. Why mtDNA rather than nuclear DNA? Because mtDNA, unlike nuclear DNA, is transmitted only through the female lineage. Similarities in mtDNA sequences thus imply descent from the same maternal ancestor.
The population sample analyzed 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. Western Africa was chosen because it is one of the few regions where both populations coexist.
The researchers identified an ancestral and autochthonous lineage of mtDNA shared by Pygmies and Bantus, suggesting that both populations were originally one, and that they started to diverge from common ancestors around 70,000 years ago. After a period of isolation, during which current phenotype differences between Pygmies and Bantu farmers accumulated, Pygmy women started marrying male Bantu farmers (but not the opposite). This trend started around 40,000 years ago, and continued until several thousand years ago. Subsequently, the Pygmy gene pool was not enriched by external gene influxes. The Bantu farmers’ gene pool, on the contrary, was enriched during the so-called “Bantu expansions,” an event corresponding to technological, demographic, and linguistic advances in the late Stone Age. “Generally speaking, variability in mtDNA among Pygmies was found to be much weaker than among Bantus, indicating that the maternal gene pool among modern Pygmies comes from a small number of common ancestors,” explains Lluis Quintana-Murci, head of the CNRS-Pasteur team.
To verify their conclusions, the researchers now plan to study nuclear DNA, notably the Y chromosome. “Nuclear DNA may be more difficult to study,” says Quintana-Murci, “but it has an undeniable advantage, as it holds our entire genome, while mtDNA carries only a small part of it.”
By comparing Pygmy and Bantu populations, the researchers’ ultimate goal is to find out how the transition to a sedentary lifestyle influenced the genetic heritage, culture, and vulnerability to pathogens of these populations. A sedentary lifestyle is often accompanied by three factors that affect pathogens: demographic growth, which allows pathogens to spread more easily; the presence of garbage, which is a vector for illness; and the presence of farm animals, which carry diseases that sometimes spread to humans.

Elias Awad

Notes :

1. Laboratoire Hôtes, vecteurs et agents infectieux: biologie et dynamique (CNRS / Institut Pasteur).
2. Laboratoire Eco-anthropologie et ethnobiologie (CNRS / Muséum national d'histoire naturelle / Université Paris-VII); Dynamique du langage (CNRS / Université Lyon-II), in collaboration with the Universities of Barcelona, Haifa, Santiago de Compostela, Yale University, the CEPH-Fondation Jean Dausset in Paris, and the CIRMF in Franceville.
3. L. Quintana-Murci et al., “Maternal traces of deep common ancestry and asymmetric gene flow between Pygmy hunter-gatherers and Bantu-speaking farmers,” PNAS, 2008.105: 1596-601.
4. Mitochondria are cellular organelles that have their own DNA and that act as power plants to provide cells with energy.

Contacts :

Lluis Quintana-Murci,
CNRS / Institut Pasteur, Paris.
quintana@pasteur.fr


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