-

As far as I know ancient Summerian and Babylonian writings conjecture that life might exist off Earth and indeed that aliens may have visted our planet. Seperating practical theorizing from mythologizing is of course difficult with ancient works. Anyhow, this places the idea of extraterrestrial life 3000 to 4000 yrs before the present day. Whether there were earlier theories I don't know. In other places where atronomy was independently invented (India, China etc.) its probably fair to assume that the idea of aliens must have arisen. Note that the five inner planets (excluding Earth) would all have appeared very unique to any society capable of looking at the stars systematically and would long have appeared good candidates for life.