"Inferring the probable locations of earlier speech communication is accomplished by applying the principle of least moves to explain the language distribution of subsequent times: the most probable earlier locations are those which require the fewest population movements or cultural-cum-linguistic expansions to account for later locations. The course of argument in such determinations begins normally with the more closely related tongues and proceeds by stages to the more distant.
"The Kir-Abbaian group provides a suitable starting point for the process of establishing probable and possible earlier locations of Nilo-Saharan communities. Previous work (Ehret 1983) has shown that the proto-Kir-Abbaian homeland is best located somewhere in the Blue Nile (Abbai) river region of the modern nation of Sudan. Kir Abbaian is one of three branches of Eastern Sudanic, the others being Astaboran and Kuliak. The modern locations of languages of the Astaboran branch - along an exis extending from Tama and its related dialects on the east side of J. Marra, to Nara of the far northwestern Ethiopian highlands - suggest an original spread of the languages of that branch out of an intermediate area, just to the north of the lands in which the earlier Kir-Abbaian speakers are most probably to be placed. In other words, the proto-Astaboran society probably lived north of the latitudes of the Blue Nile, perhaps in the plains of the Atbara region or to the west of the Nile in the same latitudes. (The name Astaboran is taken from the ancient recorded form of the name of the Atbara river.) The third branch of Eastern Sudanic, Kuliak, is composed of languages spoken today only in eastern Uganda, far to the south. But the traceability of both the other branches to adjoining areas of northern Sudan indicates that the proto-Eastern Sudanic society, from which all three derive, is also best located in northern Sudan, probably in notheastern or central parts of that country. Only one movement, southward out of that region, is then required to account for the Kuliak presence in Uganda."
"The focus of cultural and linguistic divergence in the several eras immediately preceeding proto-Eastern Sudanic times apparently lay in those portions of northern Sudan westward from the Nile. The Maban languages are concentrated in and around the Wadai region of Chad, just beyond the western border of Sudan. In addition to its Eastern Sudanic and Maban branches, the Sahelian group has two other branches which each contain only a single language today. Of these, For is spoken in the J. Marra region (Darfur) in western Sudan, just east of the Maban heartland and adjacent to the span of territory to its northeast in which the early Eastern Sudanic speech areas were most probably located. Songay alone is spoken far away from the other three branches, in areas 2000km to the west near the great bend of the Niger river. As for Eastern Sudanic, so for Sahelian can all but one of its branches thus be traced to adjoining regions, with the distant locations of its remaining branch explainable by a single movement away from the others. The proto-Sahelian country thus probably lay somewhere in or around the northwestern quarter of Sudan. The Songay presence far to the west is evidence that the break-up of the proto-Sahelian speech community was accomplished in part by a major expansion of people westward across what is today the Sahel and southern Sahara.
"These inferences about the location of the early Sahelian people are further strengthened by the evidence pertaining to the next earlier era of Nilo-Saharan differentiation, in which the Proto-Saharo-Sahelian language was spoken. The Saharo-Sahelian mother language diverged into two daughter languages, one of which was Proto-Sahelian, the homeland of which lay, as has just been proposed, in or around the northwestern parts of Sudan. The other daughter language was Proto-Saharan, from which the present day Kanuri, Tibu and Zaghawa languages derive. The work of Saxon (n.d.) has shown that the Proto-Saharan language was most probably spoken in the areas extending from Tibesti on the north to Ennedi or J. Marra on the south - in other words, in the northwestern quarter of Chad, just next to the region where its sister language, Proto-Sahelian, was most likely spoken. The proto-Saharo-Sahelian society can therefore also be placed somewhere in the combined regions of northwestern Sudan and northeastern Chad."