Anyway, the story goes Lemnoch returns home to find his wife pregnant, but he hasn't been there for more than a year. So he accuses her of cheating, which she denies. He wants to believe her, but feels she might be playing him somewhere deep inside. He goes to his father and asks what to do, well daddy believes her as he's been around to watch and hasn't seen anybody schtupping his wife... so he goes to Enoch, Lemnoch's grandfather to ask about it. Enoch says that she's been inseminated from "the heavens" and that Lemnoch should accept the child. Thus Noah is born...
The theory is, if you take Lemnoch's wife at face value (personally I think she was schtupping daddy), that aliens were doing a bit of animal husbandry, and since Noah and his descendants were the only ones with the DNA that they wanted to procreate the easiest thing to do is to kill off all the humans... Big flood.
However the aliens still need to eat, humans too so they stack up a bunch of tiny vials with enough DNA to repopulate earth with wildlife and humans of the right DNA stock.