Genetic perspectives on the origin of clicks in Bantu languages from southwestern Zambia

Abstract

Some Bantu languages spoken in southwestern Zambia and neighboring regions of Botswana, Namibia, and Angola are characterized by the presence of click consonants, whereas their closest linguistic relatives lack such clicks. As clicks are a typical feature not of the Bantu language family, but of Khoisan languages, it is highly probable that the Bantu languages in question borrowed the clicks from Khoisan languages. In this paper, we combine complete mitochondrial genome sequences from a representative sample of populations from the Western Province of Zambia speaking Bantu languages with and without clicks, with fine-scaled analyses of Y-chromosomal single nucleotide polymorphisms and short tandem repeats to investigate the prehistoric contact that led to this borrowing of click consonants. Our results reveal complex population-specific histories, with female-biased admixture from Khoisan-speaking groups associated with the incorporation of click sounds in one Bantu-speaking population, while concomitant levels of potential Khoisan admixture did not result in sound change in another. Furthermore, the lack of sequence sharing between the Bantu-speaking groups from southwestern Zambia investigated here and extant Khoisan populations provides an indication that there must have been genetic substructure in the Khoisan-speaking indigenous groups of southern Africa that did not survive until the present or has been substantially reduced.

Figure 1

Map showing the position of Zambia within the African continent and the location of the villages sampled. Thirty-one circles are plotted according to their registered latitude and longitude; populations sampled are listed with information on language affiliation and presence/absence of clicks.

Figure 2

Multi-dimensional scaling based on F_ST distances calculated from haplogroup frequencies. Stress value: 0.011. Bantu Luyana populations are indicated in italics, Bantu Botatwe in regular bold. Populations that speak languages with clicks are indicated with a square, the remaining populations with a dot.

Figure 3

Multi-dimensional scaling based on Φ_ST distances between mtDNA sequences. Stress value: 0.007. Bantu Luyana populations are indicated in italics, Bantu Botatwe in regular bold. Populations that speak languages with clicks are indicated with a square, the remaining populations with a dot.

Figure 4

Median joining network of mtDNA sequences belonging to haplogroup L0k and L0d, including individuals from Zambia and sequences retrieved from the literature; the latter are here lumped as ‘Khoisan' and ‘Bantu', respectively. Numbers on the branches indicate the number of mutations having taken place along that branch.