The Western Pygmies from the Central African Republic: new results on autosomal loci

hypotheses concerning the origin of African Pygmies, termed “recent divergence and differential admixture” (RDDA) and “pre-Bantu divergence” (PBD) (Destro-Bisol et al.,, 2004). The RDDA hypothesis sustains that the genetic differentiation observed between Eastern Pygmies (settled in Zaire) and Western Pygmies (Cameroon, Central African Republic and Congo) is due to the combined effect of differential admixture and genetic drift (see also Cavalli-Sforza, 1986). This theory suggests that the occupation of rainforest and deforestation by Bantu farmers (around 2-3 kiloyears ago) progressively reduced the Pygmies’ habitat, eventually leading to a biological separation of Pygmy groups settled in the eastern and western sides of the tropical rainforest. The Eastern Pygmies remained relatively isolated from neighbouring populations, whereas Western Pygmies admixed with Bantus. The PBD hypothesis, on the other hand, states that the separation between Eastern and Western Pygmies occurred well before 2-3 Kya, independently from the Bantu expansion. It takes into consideration some archeological studies in the tropical rainforest which indicate the presence of a hunter-gatherer community in the tropical forest of Ituri of the Northeastern Congo Basin around 18 Kya (Mercader, 2002). These sites might be the remains of the nucleus from which present-day Eastern Pygmies evolved after the separation from the Western Pygmy branch. Therefore, the 18 Kya date could be tentatively regarded as the ante quam date for the divergence between the two Pygmy groups. It is also worth noting that the Last Glacial Maximum dates back to around this time, and that there is a broad consensus sustaining that during this time the area occupied by rainforest was considerably reduced compared to today (Cornellissen, 2002). This can be considered as a possible ecological cause of separation between the two groups of Pygmies, whose present distribution resembles the probable forest refugia of the Last Glacial Maximum (Sayer et al., 1992). Destro-Bisol et al. (2004) tested the RDDA and PBD hypotheses considering the variation of mitochondrial DNA in Western Pygmies (Mbenzele and Biaka from the Central African Republic) and Eastern Pygmies (Mbuti from the Popular Republic of Congo). Some Bantu-speaking farmer populations were also considered. We observed that distribution, sequence variation, and age of haplogroups, along with genetic distances, estimates of divergence times among populations, and simulations based on the coalescent approach, met the expectations of the PBD hypothesis, whereas they do not support the RDDA hypothesis. Given that mtDNA may be regarded as a single locus in evolutionary terms, the conclusions of the study mentioned above need further checks through the use of other genetic systems. In order to do this, we here present a synthesis of new results and published data regarding autosomal variation JASs Correspondence & Notes