https://www.academia.edu/9529053/The_Ex ... l_Identity
Southern Somalia
"Before the incursions of the Hamitic Galla and Somali, this region, according to Lewis, was occupied by a mixed pre-Hamitic population-the Zengi of medieval Arab geographers-who seem to have comprised two distinct populations. Sedentary agricultural tribes, settled in the inter-riverine area survive today in Shidle, Kabole, Reer ‘Ise, Makanne, and Shabelle peoples on the Shabelle River. As Lewis (1961:22) writes:
To the same group belong the Eelaay of Baidoa in the hinterland, and the Tunni of Brava District. The other section of the pre-Hamitic population consisted of Bushmanlike hunters and gatherers, and along the rivers of fishermen, of whom contemporary representatives are the WaRibi, WaBooni of Jubaland and southern Somalia, and the Eyle of Bur Hacaba.
The Shidle, the Shabelle, and the Makanna, who are the aboriginal populations of the middle and upper Shabelle River plain, did not speak Bantu language at any known historical time (Cerulli, 1926; Cassanelli, 1982; Menkhaus, 2003). They speak the Somali language and use a considerable number of Galla-Oromo idioms. The Shidle, the Shabelle, and the Makanna are a sedentary and surely autochthonous pre-Cushitic
population, which implies, more importantly, that they are the Sab in the meaning of the term suggested by Ferrandi, which is that they are indigenous to the Horn of Africa. They maintained considerable cohesion and were powerful enough to remain politically autonomous from—and minimize predatory raids by—surrounding pastoralists (Menkhaus, 2010). The nomadic pastoralists learned the practice of agriculture from them (Puccioni, 1937). Northern pastoralists were attracted to the sedentary cultivating population of the south between the thirteenth and sixteenth centuries and even later (Cerulli, 1957). According to Cerulli, these pre-Cushitic agriculturalists along the Shabelle River are not considered low caste by the nomadic pastoralists (Cerulli, 1959). This differentiates them from the connected pre-Cushitic hunting groups such Eile, Bon, and Midgan, who are considered as low caste because of their activity. They are all indigenous pre-Somali populations and are evidently ethnically intermixed (Lewis, 1955). These groups are also those referred to as the Zenji population mentioned by Arab writers and geographers in medieval times and it seems that they consisted of two distinct groups: hunters and sedentary agriculturalists. Cerulli was aware of the autochthony of these agriculturalist populations. This is evident in Cerulli’s (1926:2-3) statements such as the following:
"I am increasingly convinced that the current explanation in relation to the undoubtedly Negro populations of the Shidle, the Shabelle etc., that is that they were the slaves of the Somalis and were liberated by their owners, is certainly to be rejected. I hold that there is no doubt that the primitive nucleus of the populations of the Shidle, the Shabelle etc. was made up of those left behind by the Negro Bantu when, under pressure from the Galla, they cleared out the people from the region of the Uebi."
According to the Book of the Zanj, noted in another post above, these groups were pushed into the upper reaches of the Jubba and other rivers by the Ajuraan/Oromo fighting during the Gaal Madow wars. These folks, called the Kasur in the book, were the aboriginal inhabitants of Somalia. Today, they are mostly assimilated among the Reewin. Despite Cerulli's opinion, they are neither Cushitic nor Bantu.