Origins of Hutu, Tutsi and Twa adults has a high capacity for lactose digestion, a surprisingly high number for an agrarian people, which Mamdani suggests may be the result of centuries of intermarriage with Tutsi. Bethwell Ogot in the 1988 UNESCO "General History" further notes that the number of pastoralists in Rwanda increased sharply around the fifteenth century. Although Luis et al. (2004) in a more general study on bi-allelic markers in many African countries found a statistically significant genetic difference between Tutsi and Hutu, the overall difference were not large. Modern-day genetic studies of the Y-chromosome generally indicate that the Tutsi, like the Hutu, are
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