How did Swahili expand as far as Eastern Congo? An account from its structural basis

The expansion of Swahili into the inner portion of Africa has routinely been explained by researchers in terms of social and economic reasons, taking into account activities of Arab traders, expansion of Islam and/or Christianity, colonisation, development of mining in Shaba region and other such factors. No explanation currently exists to my knowledge that considers the structural reasons for the expansion of Swahili, which this paper seeks to address. Concerning the power relationship between Swahili and local (mostly Bantu) languages, Swahili is thought to be on the upper level and local languages are on the lower level. This is true because Swahili is used as a lingua franca over a wide area of different languages. With respect to lexical transfer, for example, most researchers state that lexical transfer occurs from Swahili to local languages, while ignoring the influence of the local language on Swahili. However, examination of Swahili and local languages spoken together in the eastern part of the Democratic Republic of the Congo suggests that the transfer is bidirectional, and the transfer from local languages to Swahili is noteworthy. In fact, Swahili adopts elements of local languages to adapt itself to the local situation, making the language familiar to local people. This paper demonstrates this by explaining the parallelism of the tense/aspect system and kinship terminology of Congo Swahili and local languages (Tembo in particular) which differ from Standard Swahili