From the University of Cambridge, a news item:
Cambridge University manuscript specialist, Dr. Chip Coakley has identified what may be the world’s earliest example of a question mark. The symbol in question is two dots, one above the other, similar in appearance to a colon, rather than the familiar squiggle of the modern question mark. The double dot symbol appears in Syriac manuscripts of the Bible dating back to the fifth century.
…The double dot mark, known to later grammarians as zawga elaya, is written above a word near the start of a sentence to tell the reader that it is a question. It doesn’t appear on all questions: ones with a wh- word don’t need it, just as in English ‘Who is it’ can only be a question (although we use a question mark anyway). But a question like ‘You’re going away?’ needs the question mark to be understood; and in Syriac, zawga elaya marks just these otherwise ambiguous expressions.
“Reading aloud, the same function is served by a rising tone of voice – or at least it is in English – and it is interesting to ponder whether zawga elaya really marks the grammar of the question, or whether it is a direction to someone reading the Bible aloud to modulate their voice,” said Dr. Coakley.
Question marks in Greek and Latin script emerged later than in Syriac, with the earliest examples dating from the eighth century. It is likely that these symbols developed independently from each other and from Syriac. Hebrew and Arabic, close neighbours of Syriac, have nothing comparable. Armenian, another neighbour, has a similar mark, but it seems to be later.