How are Nilotic and Surmic languages historically related?

Within the Nilo-Saharan language family, both Nilotic and Surmic are classified as branches of the Eastern Sudanic languages.1

While the internal classification of the Nilo-Saharan family remains a subject of ongoing study, comparative research into lower-level units has provided evidence of a closer historical affinity between the Nilotic and Surmic groups. These languages also share certain areal features, such as a tendency toward verb-initial word order, which distinguishes them from the verb-final structures found in many western and northern Nilo-Saharan languages.23

It is important to note that while early 20th-century scholars sometimes proposed theories of language mixture—such as the "Nilo-Hamitic" hypothesis—to explain typological similarities like gender systems, modern linguistics emphasizes that genetic relationships are determined by shared innovations and correspondences in basic vocabulary and grammatical markers rather than mere typological resemblance.24