The peopling of Africa is the result of a series of migrations by large groups, proceeding over extended periods of time. Rwanda and Burundi were 1st occupied by the Batwa, a group of hunters and gatherers. The arrival of the agricultural Bahutu led to widescale clearing of the forest for agriculture. The Nilo-Hamitic Bututsi herders were the last to arrive. All the groups eventually became sedentary, and populations in some parts of Rwanda and Burundi became extremely dense. Before the colonial era, migrations were essentially internal. The number of persons of Rwandan origin not counting refugees who were living in neighboring countries is estimated to have increased from 1,919,000 in 1971 to 2,634,000 in 1980 and 3,786,000 in 1990. In 1990, 2,628,000 were in Zaire, 984,000 in Uganda, 155,000 in Tanzania, and 19,000 in Burundi. The same phenomenon has occurred in Burundi, as workers left their densely populated homes to seek work in the mines of Zaire or the cotton plantation of East Africa. Refugees from Rwanda and Burundi have consisted primarily of illiterate agricultural workers or herders who have had difficulty assimilating into the host country and dream of returning someday to their places of origin. In October 1990, there were an estimated 267,455 Rwandan refugees in Burundi, 76,294 in Zaire, 74,372 in Uganda, and 23,000 in Tanzania. There were an estimated 20,200 refugees from Burundi in Rwanda, 24,000 in Zaire, and 215,000 in Tanzania. The majority of the refugees are installed in unexploited rural areas designated by the host country and sufficiently distant from the border to discourage a violent mass return to the country of origin. Rural-rural migration is important in both Rwanda and Burundi, as surplus population leaves the fertile and healthy central areas of their countries for the more arid and less healthy areas to the east and west. Rural-urban migration is in its early stages in both countries. A main reason for migratory movement in Rwanda and Burundi has been their extraordinarily rapid population growth. Burundi's population increased from 1,524,000 in 1935 to 5,562,354 in 1991, while Rwanda's increased from 1,685,283 in 1935 to 7,844,000 in 1991. Migrations in the area occur in response to sociopolitical and economic stimuli, but they are also deeply embedded cultural practices. Data are lacking that would allow understanding of the sociodemographic effects of migration on the countries of origin and of destination. But the massive return of migrants and refugees to Rwanda would have grave consequences in a country already densely populated and with a precarious food supply.