Genetic history of Chad

Abstract

The Sahel is a semi-arid zone stretching from the Atlantic Ocean in the west to the Red Sea in the east and from the Sahara in the north to the Sudanian Savanna in the south. Here, we investigated the genetic history of the spread of Northern African ancestry common among Berbers, the Y DNA haplogroup R1b-V88, and Chadic languages throughout the Sahel, with a focus on Chad. We integrated and analyzed genotype data from 751 individuals from Chad, Burkina Faso, Mali, South Sudan, and Sudan in the context of a global reference panel of 5,966 individuals. We found that genetic diversity in Chad was broadly divided by a north-south axis. The core ancestry of Southern Chadians was Central African, most closely related to Pygmies. Southern Chadians then experienced four waves of gene flow over the last 3,000 years from West-Central Africans, Eastern Africans, West-Central Africans again, and then Arabians. In contrast, Northern Chadians did not share Central African ancestry and were not influenced by the first wave of West-Central Africans but were influenced by Northern African ancestry. We found that Y DNA haplogroup R1b entered the Chadian gene pool during Baggarization. Baggara Arabs spoke Arabic, not Chadic, implying that people carrying R1b-V88 were not responsible for the spread of Chadic languages, which may have spread approximately 3,700 years ago. We found no evidence for migration of Near Eastern farmers or any ancient episodes involving Eurasian backflow.