REMNANT OF GIANTS: The Earliest Non-Angelic Interpretation of Genesis 6.1-4.
Tyrone Slothrop thinks it's as early as Ben Sira. His interpretation of Sir 16:7 sounds possible, but I think one could also read the verse to say that God did not forgive the giants (who were superhuman but still mortal, and thus not inappropriate for this list) when they revolted in their might and filled the earth with wickedness before the Flood (Gen 6:11). As Tyrone observes, Ben Sira's phrase "in their might" alludes to Gen 6:4, which calls the Nephilim/giants, not the angelic Watchers, "mighty men." So I don't see anything in the verse that need be taken to say that the Watchers themselves (the sons of God) were human.
UPDATE (23 February): Have a look at Tyrone's post again. He has appended a response to my comments above.