Why do some Nilo-Saharan languages have sound features shared with unrelated neighboring languages?

Some Nilo-Saharan languages exhibit sound features shared with unrelated neighboring language groups due to the areal diffusion of structural properties. This phenomenon typically arises from extensive historical contact and mutual borrowing between groups in situations of long-term bilingualism.1

Rather than being inherited from a common ancestor, these shared traits—such as complex consonant systems or specific tonal patterns—are often the result of linguistic interaction. For example:

• Consonant Systems: Some Central Sudanic languages share complex consonant systems with neighboring Niger-Congo languages, while southern Nilotic languages have adopted simpler consonant systems similar to those found in neighboring Bantu languages.1
• Tone: Many Nilo-Saharan languages are tonal, but their specific tonal complexity can be influenced by proximity to other tonal or nontonal groups.2

This process involves the borrowing of not only lexical items but also fundamental structural features, reflecting the deep history of cultural and linguistic exchange across the African continent.1