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.
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.
Commentators in Arabic in the medieval period used the general term bilâd as-sûdân ("Land of the Blacks") for the vast Sudan region (an expression denoting West and Central Africa), or sometimes extending from the coast of West Africa to Western Sudan. Its equivalent in Southeast Africa was Zanj ("Country of the Blacks"), which was situated in the vicinity of the Great Lakes region.