Koen Bostoen

Y-chromosomal variation in Sub-Saharan Africa: insights into the history of Niger-Congo groups

Technological and cultural innovations as well as climate changes are thought to have influenced the diffusion of major language phyla in sub-Saharan Africa. The most widespread and the richest in diversity is the Niger-Congo phylum, thought to have originated in West Africa ∼ 10,000 years ago (ya). The expansion of Bantu languages (a family within the Niger-Congo phylum) ∼ 5,000 ya represents a major event in the past demography of the continent. Many previous studies on Y chromosomal variation in Africa associated the Bantu expansion with haplogroup E1b1a (and sometimes its sublineage E1b1a7). However, the distribution of these two lineages extends far beyond the area occupied nowadays by Bantu-speaking people, raising questions on the actual genetic structure behind this expansion. To address these issues, we directly genotyped 31 biallelic markers and 12 microsatellites on the Y chromosome in 1,195 individuals of African ancestry focusing on areas that were previously poorly characterized (Botswana, Burkina Faso, Democratic Republic of Congo, and Zambia). With the inclusion of published data, we analyzed 2,736 individuals from 26 groups representing all linguistic phyla and covering a large portion of sub-Saharan Africa. Within the Niger-Congo phylum, we ascertain for the first time differences in haplogroup composition between Bantu and non-Bantu groups via two markers (U174 and U175) on the background of haplogroup E1b1a (and E1b1a7), which were directly genotyped in our samples and for which genotypes were inferred from published data using linear discriminant analysis on short tandem repeat (STR) haplotypes. No reduction in STR diversity levels was found across the Bantu groups, suggesting the absence of serial founder effects. In addition, the homogeneity of haplogroup composition and pattern of haplotype sharing between Western and Eastern Bantu groups suggests that their expansion throughout sub-Saharan Africa reflects a rapid spread followed by backward and forward migrations. Overall, we found that linguistic affiliations played a notable role in shaping sub-Saharan African Y chromosomal diversity, although the impact of geography is clearly discernible.

Bantu expansion shows that habitat alters the route and pace of human dispersals

Significance Humans are uniquely capable of using cultural innovations to occupy a range of environments, raising the intriguing question of whether historical human migrations have followed familiar habitats or moved relatively independently of them. Beginning ∼5,000 y ago, savannah-dwelling populations of Bantu-speaking peoples swept out of West Central Africa, eventually occupying a vast geographical area. We show that this expansion avoided unfamiliar rainforest habitats by following savannah corridors that emerged from the Congo rainforest, probably from climate change. When Bantu speakers did move into the rainforest, migration rates were delayed by on average 300 y compared with similar movements on the savannah. Despite unmatched abilities to produce innovations culturally, unfamiliar habitats significantly alter the route and pace of human dispersals. Unlike most other biological species, humans can use cultural innovations to occupy a range of environments, raising the intriguing question of whether human migrations move relatively independently of habitat or show preferences for familiar ones. The Bantu expansion that swept out of West Central Africa beginning ∼5,000 y ago is one of the most influential cultural events of its kind, eventually spreading over a vast geographical area a new way of life in which farming played an increasingly important role. We use a new dated phylogeny of ∼400 Bantu languages to show that migrating Bantu-speaking populations did not expand from their ancestral homeland in a “random walk” but, rather, followed emerging savannah corridors, with rainforest habitats repeatedly imposing temporal barriers to movement. When populations did move from savannah into rainforest, rates of migration were slowed, delaying the occupation of the rainforest by on average 300 y, compared with similar migratory movements exclusively within savannah or within rainforest by established rainforest populations. Despite unmatched abilities to produce innovations culturally, unfamiliar habitats significantly alter the route and pace of human dispersals.

Bringing together linguistic and genetic evidence to test the Bantu expansion

The expansion of Bantu languages represents one of the most momentous events in the history of Africa. While it is well accepted that Bantu languages spread from their homeland (Cameroon/Nigeria) approximately 5000 years ago (ya), there is no consensus about the timing and geographical routes underlying this expansion. Two main models of Bantu expansion have been suggested: The ‘early-split’ model claims that the most recent ancestor of Eastern languages expanded north of the rainforest towards the Great Lakes region approximately 4000 ya, while the ‘late-split’ model proposes that Eastern languages diversified from Western languages south of the rainforest approximately 2000 ya. Furthermore, it is unclear whether the language dispersal was coupled with the movement of people, raising the question of language shift versus demic diffusion. We use a novel approach taking into account both the spatial and temporal predictions of the two models and formally test these predictions with linguistic and genetic data. Our results show evidence for a demic diffusion in the genetic data, which is confirmed by the correlations between genetic and linguistic distances. While there is little support for the early-split model, the late-split model shows a relatively good fit to the data. Our analyses demonstrate that subsequent contact among languages/populations strongly affected the signal of the initial migration via isolation by distance.

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

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.

Molecular perspectives on the Bantu expansion: a synthesis

The expansion of Bantu-speaking peoples over large parts of sub-Saharan Africa is still a matter of debate—not only with respect to the propelling force behind it and the route(s) taken, but, also, in terms of the question whether there actually was a demographic expansion of peoples, rather than just a cultural expansion involving the spread of languages and technologies. In this paper, we provide a critical review of the extant linguistic and molecular anthropological data on Africa and discuss the insights they provide concerning the expansion itself as well as the demographic processes involved in it. Contrary to some assumptions by historians and cultural anthropologists, the genetic data speak in favor of an actual movement of peoples during the expansion of the Bantu languages over Africa, rather than a spread through language and culture shift. Furthermore, the molecular data indicate that sociocultural practices such as patrilocality and possibly even polygyny played a role in shaping the genetic diversity of Bantu-speaking peoples. These sociocultural practices might explain why, in Africa, there is a correlation between Y-chromosomal (i.e., paternal) lineages and linguistic affiliation, but not between mtDNA (maternal) lineages and language.

Middle to Late Holocene Paleoclimatic Change and the Early Bantu Expansion in the Rain Forests of Western Central Africa

This article reviews evidence from biogeography, palynology, geology, historical linguistics, and archaeology and presents a new synthesis of the paleoclimatic context in which the early Bantu expansion took place. Paleoenvironmental data indicate that a climate crisis affected the Central African forest block during the Holocene, first on its periphery around 4000 BP and later at its core around 2500 BP. We argue here that both phases had an impact on the Bantu expansion but in different ways. The climate-induced extension of savannas in the Sanaga-Mbam confluence area around 4000–3500 BP facilitated the settlement of early Bantu-speech communities in the region of Yaoundé but did not lead to a large-scale geographic expansion of Bantu-speaking village communities in Central Africa. An extensive and rapid expansion of Bantu-speech communities, along with the dispersal of cereal cultivation and metallurgy, occurred only when the core of the Central African forest block was affected around 2500 BP. We claim that the Sangha River interval in particular constituted an important corridor of Bantu expansion. With this interdisciplinary review, we substantially deepen and revise earlier hypotheses linking the Bantu expansion with climate-induced forest openings around 3000 BP.

Early Plant Cultivation in the Central African Rain Forest: First Millennium BC Pearl Millet from South Cameroon

The Bantu expansion, a major topic in African archaeology and history, is widely assumed to correlate with the spread of farming, but archaeological data on the subsistence of these putative early Bantu speakers are very sparse. However, finds of domesticated pearl millet (Pennisetum glaucum) in southern Cameroonian archaeological sites, dated between 400 and 200 BC, open new perspectives on the history of agriculture in the Central African rain forest.Linguistic evidence suggests that pearl millet was part of early agricultural traditions of Bantu speakers, and has to a great extent been distributed during the course of their expansion over large parts of western Bantu-speaking Africa, possibly even originally from their homeland in the Nigerian-Cameroonian borderland.In combining archaeobotanical, palaeoenvironmental and linguistic data, we put forward the hypothesis that an agricultural system with pearl millet was brought into the rain forest during the first millennium BC, and that its spread across Central Africa coincided with the dispersal of certain Bantu language subgroups.

Migration and interaction in a contact zone: mtDNA variation among Bantu-speakers in Southern Africa

Bantu speech communities expanded over large parts of sub-Saharan Africa within the last 4000–5000 years, reaching different parts of southern Africa 1200–2000 years ago. The Bantu languages subdivide in several major branches, with languages belonging to the Eastern and Western Bantu branches spreading over large parts of Central, Eastern, and Southern Africa. There is still debate whether this linguistic divide is correlated with a genetic distinction between Eastern and Western Bantu speakers. During their expansion, Bantu speakers would have come into contact with diverse local populations, such as the Khoisan hunter-gatherers and pastoralists of southern Africa, with whom they may have intermarried. In this study, we analyze complete mtDNA genome sequences from over 900 Bantu-speaking individuals from Angola, Zambia, Namibia, and Botswana to investigate the demographic processes at play during the last stages of the Bantu expansion. Our results show that most of these Bantu-speaking populations are genetically very homogenous, with no genetic division between speakers of Eastern and Western Bantu languages. Most of the mtDNA diversity in our dataset is due to different degrees of admixture with autochthonous populations. Only the pastoralist Himba and Herero stand out due to high frequencies of particular L3f and L3d lineages; the latter are also found in the neighboring Damara, who speak a Khoisan language and were foragers and small-stock herders. In contrast, the close cultural and linguistic relatives of the Herero and Himba, the Kuvale, are genetically similar to other Bantu-speakers. Nevertheless, as demonstrated by resampling tests, the genetic divergence of Herero, Himba, and Kuvale is compatible with a common shared ancestry with high levels of drift, while the similarity of the Herero, Himba, and Damara probably reflects admixture, as also suggested by linguistic analyses.

Combined Spectroscopic Analysis of Beads from the Tombs of Kindoki, Lower Congo Province (Democratic Republic of the Congo)

Raman spectroscopy and X-ray fluorescence (XRF) analysis are commonly applied to archaeological objects as a fast and nondestructive way to characterize the materials. Here, micro-Raman spectroscopy and chemometrics on handheld XRF results were used to completely characterize beads found during archaeological excavations in the Congo. Metallic objects, organogenic materials, and glass beads were studied. Special attention was paid to the glassy materials, as they seem to be of European production. The matrix family and crystalline phases assemblage, as well as the results from principal components analysis on the elemental data, were used to define groups of beads of similar composition, and therefore probably of similar origin. This research project establishes the feasibility of this approach to archaeological glasses, and can be used to confirm and support the bead typologies used by archaeologists.

The causative/applicative syncretism in Mbuun (Bantu B87, DRC): Semantic split or phonemic merger?

Abstract In Mbuun (B87), a Bantu language spoken in the Bandundu Province of the Democratic Republic of the Congo, the valency-increasing functions of the causative and the applicative have, to a certain extent, the same formal expression. They are encoded morphologically by a derivational suffix which has an uncharacteristic morphophonological realization. It triggers the gemination of the root-final consonant. In this paper, we examine whether this causative/applicative isomorphism is the result of a semantic split or rather the outcome of a phonemic merger. In the first case, the semantic extension must have happened from applicative to causative rather than the cross-linguistically more common evolution from causative to applicative, since the consonant gemination associated with these forms has its origin in the Proto-Bantu applicative suffix *- i d-. In the case of a phonemic merger, however, this applicative suffix would have become homophonous with an originally distinct suffix having a causative effect as the result of convergent morphophonological change. This morpheme cannot be the Proto-Bantu causative suffix *-ici-, which still has a residual causative use in Mbuun.