Hutu Rwanda, this led to the "Social revolution" and Hutu violence against Tutsis. Tens of thousands of Tutsis were killed and many others fled to neighboring countries, such as Burundi, Uganda and expanding the Banyamulenge Tutsi ethnic group in the South Kivu region of the Belgian Congo. Later, exiled Tutsis from Burundi invaded Rwanda, prompting Rwanda to close its border with Burundi. In Burundi, a campaign of genocide was conducted against Hutu population in 1972, and an estimated 100,000 Hutus died. In 1993, Burundi's first democratically elected president, Melchior Ndadaye, who was Hutu, was believed to be assassinated by Tutsi officers,
Origins of Hutu, Tutsi and Twa communities (17.7% Afro-Asiatic genes found in the mixed Hutu/Tutsi population). While most supporters of the migration theory are also supporters of the "Hamitic theory", namely that the Tutsi came from the Horn of Africa, a later theory proposed that the Tutsi had instead migrated from nearby interior East Africa, and that the physical differences were the result of natural selection in a dry arid climate over millennia. Among the most detailed theories was one put forward by Jean Hiernaux, based on studies of blood factors and archeology. Noting the fossil record of a tall people with narrow facial features several