<P> The sub-Saharan Africa is, geographically, the area of the continent of Africa that lies south of the Sahara . According to the United Nations, it consists of all African countries that are fully or partially located south of the Sahara . It contrasts with North Africa, whose territories are part of the League of Arab states within the Arab world . The Arabic speaking states of Somalia, Djibouti, Comoros and the Arabic speaking Mauritania are however geographically in Sub-Saharan Africa, allthough they are members of the Arab league as well . </P> <P> The Sahel is the transitional zone in between the Sahara and the tropical savanna of the Sudan region and farther south the forest - savanna mosaic of tropical Africa . </P> <P> Since probably 3500 BCE, the Saharan and Sub-Saharan regions of Africa have been separated by the extremely harsh climate of the sparsely populated Sahara, forming an effective barrier interrupted by only the Nile in Sudan, though the Nile was blocked by the river's cataracts . The Sahara pump theory explains how flora and fauna (including Homo sapiens) left Africa to penetrate the Middle East and beyond . African pluvial periods are associated with a Wet Sahara phase, during which larger lakes and more rivers existed . </P> <P> The use of the term has been criticized because it refers to the South only by cartography conventions and projects a connotation of inferiority; a vestige of colonialism, which some say, divided Africa into European terms of homogeneity . </P>

What geographical feature limited contact between sub-saharan africa and the outside world