From around the eighth millennium BCE a group of cultures developed along the lakes and rivers which extended from Lake Rudolf in East Africa, along the upper reaches of the Nile, and across to the bend in the Niger River in West Africa. These cultures were predominantly related through a common agricultural system based on the exploitation of river and lake food resources. From the sixth millennium BCE the region was increasingly affected by the desertification of the Sahara to the north, resulting in the isolation of several groups across the continent. It is believed that this isolation explains the linguistic drift now identified in the various members of the Nilo-Saharan language group.
In the west these groups are associated with kingdoms, states and empires which formed the southern end of trans-Saharan trade routes. Songhai, for example, which is now spoken by upwards of one million people, is the language of a once expansive African empire which stretched for several thousand kilometres along the Niger River.
The Saharan branch of the language group is still spoken in north-eastern Nigeria, across into Chad and north into the oasis settlements of southern Libya. Kanuri, the most predominant of the Saharan branch is spoken by upwards of one-and-a-half million people today.
From the northern end of Chad, across The Sudan, down into Uganda and Kenya, and across into the northern end of the DRC, is a another branch of Nilo-Saharan language group known as Chari-Nile. This includes the Nilotic language sub-group of Dinka, Nuer, Shilluk, and Acholi (also known as Luo).
The Nilo-Saharan language group was originally defined in 1963 by the American linguist and anthropologist Joseph Greenberg. Of the four major language groups identified in Africa (the others being Afro-Asiatic, Niger-Congo, and KhoiSan) Nilo-Saharan is the most controversial. Debate continues today as to whether it is actually a language group in its own right, or a sub-group of the wider spread Niger-Congo group. Of the four language groups it exhibits the largest linguistic drift, and is often used to perpetuate the Euro-centric explanation for the spread of iron working from north Africa through trade rather than independent development by sub-Saharan cultures.