Jonathan Ben-Dov (University of Haifa) has developed a very interesting theory about the origin of the early Jewish tradition of the Fallen Angels and Giants. He presented it at Boston College on November 20, 2013, in a paper entitled “Iconography and Myth: From Nebuchadnezzar to the Fallen Angels”.
Ben-Dov’s theory is that the Watcher tradition derives, at least in part, from an attempt to interpret the gigantic iconography of great Mesopotamian kings on Assyrian, Babylonian, and Achaemenid monuments and reliefs. This was carried out in the third or second century BCE, long after the inscriptions were made, by which time they were not properly understood.
In his presentation, Ben-Dov examines many of the elements shared, on the one hand, by Mesopotamian inscriptions and monuments and, on the other hand, by Jewish literature such as 1 Enoch, the Book of Giants, Jubilees, etc.
In one part of his presentation, Ben-Dov discusses this interesting passage from Jubilees, which provides an example of a later generation (mis)interpreting an ancient inscription as referring to the Watchers:
And he [Kainam] found a writing which former (generations) had carved on the rock, and he read what was thereon, and he transcribed it and sinned owing to it; for it contained the teaching of the Watchers in accordance with which they used to observe the omens of the sun and moon and stars in all the signs of heaven. And he wrote it down and said nothing regarding it; for he was afraid to speak to Noah about it lest he should be angry with him on account of it. (Jubilees 8.3-4)
Ben-Dov also discusses inscriptions left by Nebuchadnezzar in Lebanon, studied recently by Rocío Da Riva.
Ben-Dov’s theory is well worth consideration, as identifying one of the concrete contributing causes of the Watcher tradition. I don’t know, due to the nature of the evidence, whether his theory could ever be conclusively established, but I do think it is at least plausible that the reliefs and inscriptions contributed to the construction of the Watcher myth. Have a watch of his lecture here.
Note also Brian Doak’s discussion of depictions of gigantic figures in ancient Near Eastern iconography in his The Last of the Rephaim, pp. 14-16.
Update (10 January 2016): Jim Davila notes that Ben-Dov’s theory was also set out in an article in Haaretz on October 18, 2013, “Turning to the Angels to Save Jewish Mythology” (subscription required).