Genetic history of the African Sahelian populations

Abstract

From a biogeographic perspective, Africa is subdivided into distinct horizontal belts. Human populations living along the Sahel/Savannah belt south of the Sahara desert have often been overshadowed by extensive studies focusing on other African populations such as hunter-gatherers or Bantu in particular. However, the Sahel together with the Savannah bordering it in the south is a challenging region where people had and still have to cope with harsh climatic conditions and show resilient behaviours. Besides exponentially growing urban populations, several local groups leading various lifestyles and speaking languages belonging to three main linguistic families still live in rural localities across that region today. Thanks to several years of consistent population sampling throughout this area, the genetic history of the African Sahelian populations has been largely reconstructed and a deeper knowledge has been acquired regarding their adaptation to peculiar environments and/or subsistence modes. Distinct exposures to pathogens-in particular, malaria-likely contributed to their genetic differentiation for HLA genes. In addition, although food-producing strategies spread within the Sahel/Savannah belt relatively recently, during the last five millennia according to recent archaeological and archaeobotanical studies, remarkable amounts of genetic differences are also observed between sedentary farmers and more mobile pastoralists at multiple neutral and selected loci, reflecting both demographic effects and genetic adaptations to distinct cultural traits, such as dietary habits.

Demographic and Selection Histories of Populations Across the Sahel/Savannah Belt

“…S10–S12, Supplementary Material online). In agreement with previous studies ( Triska et al 2015 ; Hollfelder et al 2017 ; Černý et al 2018 ), Arabic-speaking populations across the Central and Eastern Sahel have an important genetic contribution from populations with Middle Eastern-related ancestry and complex admixture dynamics with African groups ( supplementary table S4, Supplementary Material online), such as the Baggara Arab population residing in Chad and Sudan (on average 27.6% ± 15.6 SD and 20.5% ± 9.1 SD, respectively) and the Kababish Arab population from Sudan (39.0% ± 16.9 SD). The Rashaayda Arab population from Sudan has the highest values for Middle Eastern-related ancestry (95.1% ± 4.0 SD), which is even higher than in the studied populations from Yemen or Lebanon (ranging from 57.3% ± 1.4 SD to 75.8% ± 4.5 SD).…”

“…It has been shown that the genetic differentiation among pastoralists and farmers does not represent significant population structure and that intensive gene-flow or shared ancestry might be among the possible reasons for this ( Černý et al 2021 ). There is, however, an asymmetry in maternal gene-flow between sedentary farmers and Fulani pastoralists in the western part and between sedentary farmers and Arabic pastoralists in the eastern part of the Sahel belt ( Černý et al 2018 ). While the Fulani continually loose inherent mitochondrial-DNA (mtDNA) lineages (Fulani women usually marry neighboring farmers), Arab groups, who originally arrived from the Arabian Peninsula, more often accepted women from local sub-Saharan populations into their communities ( Čížková et al 2017 ).…”

Population history and genetic adaptation of the Fulani nomads: inferences from genome-wide data and the lactase persistence trait

“…The presence of this “European” LP variant at relatively high frequencies across different Fulani populations is puzzling and could either result from convergent evolution in both Africa and Europe or from gene flow between ancestors of the Fulani and Europeans. The later hypothesis is supported by the fact that T-13910 has not been detected (or is only present at very low frequencies) in neighbouring populations of the Fulani [29, 37] and that European admixture in Fulani genomes has been reported in previous studies [17, 38].…”

“…This variant is generally not detected in most East African and Middle Eastern populations, where other LP variants are observed instead [29–31, 33, 34]. Fulani populations living mainly in the western Sahel/Savannah belt, however, carry the European-LP mutation with frequencies ranging from 18 to 60% [29, 35–37]. The presence of this “European” LP variant at relatively high frequencies across different Fulani populations is puzzling and could either result from convergent evolution in both Africa and Europe or from gene flow between ancestors of the Fulani and Europeans.…”

Subsistence strategy was the main factor driving population differentiation in the bidirectional corridor of the African Sahel

“…who live mainly in the western and central part of the Sahel, and Arabic‐speaking nomads, called Baggara in Sudan and eastern Chad and Shuwa around Lake Chad, who live mainly in the eastern part of the Sahel (Homewood, ). The main contact zone between these two groups is around Lake Chad, which acts as a boundary of the Sahel/Sudan bidirectional corridor (Černý et al, ; Černý, Salas, Hájek, Žaloudková, & Brdička, ). The Fulani are an autochthonous sub‐Saharan population (Čížková et al ), who mixed with population(s) of Eurasian ancestry in the late Holocene (Kulichová et al, ; Triska et al, ), while Sahelian Arabic‐speaking nomads are a Eurasian population which spread from the Arabian Peninsula from seventh century CE onwards (Zeltner, ), arrived in the Lake Chad region in the 14th century (Levy & Holl, ), and mixed to a high degree with various sub‐Saharan populations (Priehodová et al, ).…”