"Appears suddenly about 1575, and is exceedingly common about 1600." Probably from use of aye (“ever, always”) as expression of agreement or affirmation, or from Middle English a ye (“oh yes”), or synthesis of both. Compare Faroese ája (“certainly, ah yes”). More at oh, yea. Online Etymology Dictionary also with these posits a possible descent from I (as if clipped from e.g. "I assent").