Since the first thread on this has degenerated into a lopsided, though passionate, debate on unorthodox theories of the origin of civilizations, I’m trying to get back to the issue as discussed in this Staff Report, and adding some ramblings of my own…
… not really on the Nephilim, but rather on the “the sons of heaven taking the daughters of men for wives” issue. (As the report says, it’s by no means clear that the race of “Giants, who were the Heroes of old” are to be understood as the offspring of this union; but rather Genesis seems to claim that the miscegenation helped humanity decay into wickedness)
There is a possible interpretation that could tie this in with the “where did Cain’s wife, and the population with which to found a city, come from” (and the "against whom would Cain have to get a protective mark, anyway?) question…
That would involve the old “multiple creations” school – if we’re to adopt a reading of the Bible where the story is claiming to be entirely literally true, but you must fill in some blanks, it could go like…:
That there were already populations of humans on Earth, as a result of the First Creation (sixth day, simultaneous male and female, on the 7th He rests, etc.);
That in the second creation, God specifically made THE lineage of humans who will play the leading roles in the spiritual show (Adam, naming the animals, the rib, Eve, Eden, etc.);
That as Genesis does not say explicitly that First Creation was destroyed, we fill that blank with an assumption that this applies only to the Land of Eden (Mesopotamia) and elsewhere on Earth, First Creation survives;
That the daughters “of men” are the first-creation humans, and the sons “of heaven” are the second-creation humans;
That the Flood ensured that all surviving humanity is descended from the Adamic lineage, thru Noah.
If we take the Bible as divinely-inspired-but-symbolic, you could attempt to shoehorn in an evolutionary parallel: the First Creation is humans having evolved naturally; the Second Creation is God taking a sample of the preexisting human animal and “breathing soul into him”, evolving him into a spiritual being; the Flood is some sort of cataclism that wipes out all other lineages.
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All in all, it’s easier to just say, “HEY, it’s a good story and a morality tale, let’s leave it at that!”
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BTW, my WAG is that the “giants who walked the Earth” refers to the people of the Fertile Crescent coming upon evidence that at some point in the past there were other great and powerful cultures, which were now gone. Many cultures have the concept of a past Age of Heroes, anyway.