How are number-marking suffixes evidence of a common Nilo-Saharan ancestor?

The use of number-marking suffixes serves as a key piece of evidence for the common historical origin of Nilo-Saharan languages.

Many languages within this family employ complex systems where nouns are marked for number—singular, plural, or collective—using grammatical elements that follow the root of the word. These systems often feature replacement patterns, where both singular and plural forms are explicitly marked by these suffixes. Because several of these number-marking suffixes appear with similar or identical forms across a wide variety of Nilo-Saharan languages, linguists conclude that these morphological features likely descend from a common ancestral language.1