Demographic History and Adaptation in African Populations

Sammanfattning: Africa is the continent where modern humans originated and yet, African demographic history remains largely unknown. Through analyzing the genetic composition of extant and extinct individuals, it is possible to reveal signals of past demographic history and adaptation. In this thesis, I applied population genetic methods to investigate both deep African history and demographic changes associated with the migrations of farmers in Africa. While Paper I and II assess the genomic landscape before the arrival farming groups, Paper III, IV and V focus on the demographic patterns associated with the emergence of various African agro-pastoral societies and how shifts in ways of subsistence resulted in different selective pressures on the genomic level. The genomes from Southern African San hunter-gatherers harbor the earliest diverging lineages and represent the first population divergence event within the modern human phylogeny. However, gene-flow from farming groups has complicated the investigation of genetic relationships between different San groups. In Paper I, I established that Southern African hunter-gatherer genetic diversity fitted an isolation-by-distance model when genomic segments that trace their ancestry to farming groups were excluded. Paper II confirmed that all extant Southern African hunter-gatherers received admixture from farming groups, through comparison with ancient DNA data from three 2,000-year-old southern African Stone Age individuals. New date estimates for the first population divergence event in the modern human phylogeny, based on the Stone Age individuals, coincided with a period in the fossil record associated with transition between archaic humans into anatomically modern humans. Paper III assesses the genetic variation of four ancient Iron Age women from current-day South Africa. I was able to further refine their genetic profiles, which were closest related to southeast Bantu-speaking farmers from current-day South Africa. In Paper IV, I propose that the emergence of pastoralism in Southern Africa arrived through a male-driven migration of East African Afro-Asiatic related groups, who introduced their pastoral subsistence practices and livestock into Southern Africa. In Paper V, I investigated the history of the Fulani population and demonstrated how a shift in subsistence practice triggered different selective pressures in the Fulani. The pastoral Fulani from the Western Sahel show relatively high frequencies of the European-associated Lactase Persistence (LP) variant. Here, I propose that the LP mutation were introduced into Fulani genomes through contact with a North African group(s) who themselves carried European admixture. Additionally, by performing the first genome wide association study (GWAS) on the lactose digestion phenotype, I confirmed the association with the MCM6/LCT locus and identified a possible association between glycemic levels after lactose intake and the SPRY2 gene. Furthermore, in addition to the LP trait, I also identified other potential signals of local adaption related to the pastoralism lifeway of the Fulani. This thesis provided further insights on how the African genomic landscape was shaped through time, influenced by the environment, interactions between different groups and adaptations to different lifeways.