How does long-term bilingualism influence the grammar and sounds of Nilo-Saharan languages?

Long-term bilingualism significantly influences Nilo-Saharan languages through the areal diffusion of structural features, leading to the sharing of grammatical and phonological properties with unrelated neighboring language groups.1

In situations of extensive historical contact and mutual borrowing, these languages often adopt structural characteristics from their neighbors rather than just lexical items. This influence is particularly evident in their sound systems:1

• Complex Consonant Systems: Some Nilo-Saharan languages, such as Koma, have developed complex consonant systems—including distinctions like voiceless ejectives versus voiced implosives—that are comparable to those found in neighboring Omotic (Afro-Asiatic) languages. Similarly, several Central Sudanic languages share complex consonant systems with adjacent Adamawa-Ubangi (Niger-Congo) languages.1
• Simplified Systems: Conversely, southern representatives of the Nilotic languages exhibit relatively simple consonant systems, mirroring the patterns common in neighboring Bantu languages.1

These shifts demonstrate how prolonged linguistic interaction can reshape the fundamental structural properties of a language family, often blurring typological boundaries between genetically distinct groups.1