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
Rwandan genocide or simply having a "Tutsi appearance". The RPF was making slow but steady gains in the north and east of the country, ending the killings in each area occupied. The genocide was effectively ended during April in areas of Ruhengeri, Byumba, Kibungo and Kigali provinces. The killings ceased during April in the "akazu" heartlands of western Ruhengeri and Gisenyi, as almost every Tutsi had been eliminated. Large numbers of Hutu in the RPF conquered areas fled, fearing retribution for the genocide; 500,000 Kibungo residents walked over the bridge at Rusumo Falls, into Tanzania, in a few days at the end