Across the African continent one finds a range of expansion zones, such as the area covered by (Narrow) Bantu languages within Niger-Congo, or the Nilotic family within Nilo-Saharan. In addition, several accretion (residual) zones can be identified. One such zone is found in the Nuba Mountains in Sudan, an area roughly the size of Scotland, where more than 40 languages are spoken, belonging to three different families: Niger-Congo, Nilo-Saharan, and Kadu. The genetic diversity in this area corresponds to an equally high degree of typological disparity. The influx of Arabic borrowings in languages in the area is a recent development resulting from the expanding role of Sudanese Arabic, as a result of which several of these languages are now endangered. Apart from this, there is very little evidence for the diffusion of lexical or grammatical features between languages in the Nuba Mountains area. Of course, this does not preclude the occasional lexical borrowing, as shown by Schneider-Blum (2013) in her Tima-English dictionary.

As argued below, this constellation of languages, manifesting dramatic genetic and typological differences in spite of their geographical proximity, is the result of historical periods of climate change in the area, as well as technological conditions, but above all it reflects language ideologies in Nuba Mountain speech communities, which are characterized by largely autarkic economies without trading networks or regional chieftaincies, where linguistic exogamy is avoided.