Illiterate Authors?

Today I started reading Bridget Gilfillan Upton’s book Hearing Mark’s Endings (Leiden: Brill, 2006). It connects up both with my interest in the end of Mark and in oral tradition.

At one point she writes the following about the author of the Gospel of Mark: “That the evangelist could read and writer Greek suggests that he had received at least a primary education, and therefore must have come from a background which offered him a certain level of privilege” (p.10).

As I read those words, I almost immediately wondered whether such an assumption is justified. We know that Paul’s letters were composed by Paul but not, as a rule at least, written by him. Perhaps merely on the basis of the content and style, we could draw conclusions about Paul’s literacy. However, Mark’s Gospel is characterized by features typical of oral storytelling. Is it not a genuine possibility that its author was only fluent as a composer and teller of orally-transmitted materials, and in order to compose a book, needed to employ a scribe?

On what basis might we conclude that an “author” in the ancient world could read and/or write? Does anyone know of a clear instance of an illiterate author, ancient or modern?