Halliday & Matthiessen (1999: 513):
[A] significant step that took place in human grammars … was … the evolution of common nouns — or rather, of common words, since verbs are “common” in this sense: that is, words denoting classes rather than individuals. It is usually assumed that these evolved out of “proper” words, prototypically the names of individual persons; the ontogenetic evidence suggests that this is one source but not the only one, another source being rather in the interpersonal function.