The hypothesis is this: the figure the Egyptians worshipped as Osiris — green-skinned lord of the dead, civilization-bringer, slain by his jealous brother and rising again — preserves in cult form the memory of the antediluvian builder. Cain.
Not Cain the literal man, but the line of Cain — the seed that learned metallurgy and music and urban building from the serpent's instruction in the field east of Eden, that survived the flood through whatever means it survived, and that resurfaced in the post-diluvian world as the architect of empire, the deifier of ancestors, the keeper of mysteries.
Cain gave up on Abel. But Abel never gave up on Cain. That is the second half of the thesis, and the half that makes everything else bearable.