Origins of Hutu, Tutsi and Twa of neighboring people, it was almost non-existent among Rwandan Tutsi. Presence of the sickle cell trait is evidence of survival in the presence of malaria over many centuries, suggesting differing origins. Regional studies of the ability to digest lactose are also supportive. The ability to digest lactose among adults is widespread only among desert-dwelling nomadic groups that have depended upon milk for millennia. Three quarters of the adult Tutsi of Rwanda and Burundi have a high ability to digest lactose, while only 5% of the adults of the neighboring Shi people of eastern Congo can. Among Hutu, one in three
History of Rwanda dominated by the majority Hutu under President Grégoire Kayibanda. Unsettled ethnic and political tensions were worsened when Juvénal Habyarimana, who was also Hutu, seized power in 1973. In 1990, the Rwandan Patriotic Front (RPF), a rebel group composed of 10,000 Tutsi refugees from previous decades of unrest, invaded the country, starting the Rwandan Civil War. The war ground on, worsening ethnic tensions, as the Hutu feared losing their gains. The assassination of Habyarimana was the catalyst for the eruption of the 1994 genocide, in which hundreds of thousands of Tutsis and some moderate Hutus were killed. The Tutsi RPF conquered