The role of nomadic pastoralists in the genetic history of the Sahelian/Savannah inhabitants
The African Sahel is a vast area delimited by the Sahara in the north, tropical rainforests in the south, the Atlantic Ocean in the west and the Red Sea in the east. Lake Chad Basin at its centre is an important natural and culturally diverse area that also divides the region into western and eastern parts.
Today, two groups with distinct food-producing subsistence lifestyles coexist across the entire length of the Sahel: nomadic pastoralists and sedentary farmers. The pastoralists form rather homogenous language groups (the Fulani in the west and Arabs in the east being the most numerous), while the sedentary farmers, with few exceptions, form rather small populations. The transition from the hunter-gatherer to agro-pastoral lifestyle occurred probably about 5,000 years ago, in relation to climatic and environmental changes. Gradual separation of the nomads from the sedentary populations was slightly more recent and began about 4,000 years ago.
The research team studied the maternal genetic structure and demographic history of the Sahel, divided from a geographical point of view into western, central and eastern parts. They analysed almost 2,000 samples from the whole region (the majority being samples collected by the team themselves) in terms of the D-loop, a highly variable segment of mitochondrial DNA (which is inherited exclusively in the maternal lineage). It should be noted that previous studies focused only on the central part of the Sahel and a much shorter section of the D-loop. The researchers investigated ten nomadic (both previously or still practicing this way of life) and 26 sedentary populations belonging to one of three linguistic groups (Nilo-Saharan, Afro-Asiatic and Niger-Congo). Out of the almost 1,994 sequences studied, they defined 1,352 unique haplotypes (i.e. variants of the analysed DNA segment).
As in previous results, the team found a lower diversity and therefore higher genetic isolation or more recent origins in the pastoralists’ populations than in farmers in the central Sahel (Lake Chad Basin). They pointed to possible differences in mating strategies between the pastoralists of the western and eastern Sahel. This is supported by the higher gene flow between the Arabic pastoralists and neighbouring farmers in the east than between the Fulani pastoralists and the sedentary populations in the west. As the team observed the maternally inherited mitochondrial data, they could hypothesise on certain regional asymmetry based on the results. In the west, the Fulani pastoralist females apparently were not mixing with other/neighbouring populations, while females from the sedentary farmer’s populations of the eastern Sahel were mixing with the newly immigrating Arab population.
A global evaluation and inclusion of archaeological and linguistic data indicates that the genetic differentiation of the Fulani pastoralists from the standard African agropastoral gene pool occurred approximately at the same period, when the Arabic pastoralists arrived in eastern Africa (7th century).
The study is an example of life sciences methods being used in humanistic sciences, such as archaeology, ethnology or linguistics.
Čížková, M., Munclinger, P., Diallo, M. Y., Kulichová, I., Mokhtar, M. G., Dème, A., … Černý, V. (2017). Genetic Structure of the Western and Eastern African Sahel/Savannah Belt and the Role of Nomadic Pastoralists as Inferred from the Variation of D-Loop Mitochondrial DNA Sequences. Human Biology, 89(4), 281–302.