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The Peopling of the Last Green Sahara

The history of human migrations across the Sahara Desert can be followed not only through the archaeological record provided by ancient settlements in the area, but also through the human genome.

An international research team, coordinated by Prof. Fulvio Cruciani from the “Charles Darwin” Department of Biology and Biotechnology at Sapienza University, has revealed how the male gene pool of North African and Sub-Saharan populations was influenced by ancient human migrations across the desert.

The study, published on Genome Biology, provides important new information to the understanding of human evolution and, in particular, to the role that the “Green Sahara” played in the peopling of Africa.

During the Holocene Climactic Optimum (12,000 – 5,000 years ago), the Sahara was a fertile stretch of land – the “Green Sahara” – that did not constitute a geographical barrier to human migrations between Sub-Saharan Africa and the Mediterranean coasts of the continent. To analyse the peopling of the Sahara, the research team used an innovative technique ((next-generation sequencing) to sequence nearly 3.3 million human Y-chromosome bases in 104 male individuals, selected by screening thousands of samples.

The study of the geographic distribution of the different Y-chromosomes allowed the team to infer past demographic events that affected our species. The analysis revealed 5966 genic variants (51% of which has never been identified before). By studying these variants in 145 African and Euro-African populations, the researchers concluded that massive human migrations took place both across the Sahara (before it became a desert) and the Mediterranean Basin.

“The Y-chromosome,” explains Eugenia D’Atanasio, the first author of the shared study, “is transmitted only to male sons by their fathers, providing an all-male perspective of recent human evolution. The comparison of this data with that provided by mitochondrial DNA (transmitted maternally) and autosomes (transmitted by both parents) indicates how the different sexes affected genetic variability in North Africa. Women influenced the more recent Arabic slave trade, while the male traits mainly affected the more ancient migrations across the Sahara.

“This analysis,” explains co-author Beniamino Trombetta, “has also revealed the massive migrations that took place across the Mediterannean basin, which involved ancient migrations from Africa to Europe, as well as Europe to Africa, something that has occurred since prehistoric times.”

Moreover, and for the first time, the study has identified genetic foundations for the human migrations across the Sahara that have only been hypothesised by analyzing material culture. This new knowledge will allow us to better understand recent human evolution and will open up new lines of research into human history.