“And the days of Adam after he had begotten Seth were eight hundred years: and he begat sons and daughters:”
Many people seem to have a problem with what must have happened during the world’s second generation. I have been asked, “Where did all the people of the world come from if Adam only had two children, and one of them murdered the other?” The purpose of such questions is to imply that Adam was either figurative or that there must have been other people in the world as well.
In fact, Genesis names three sons of Adam and Eve, not two. There was Cain, Abel, and, later, Seth. But this still doesn’t help these questioners because Seth was another son.
Apart from Cain, Abel and Seth, there were other sons who were not named, and there were daughters. So the third generation would have come from marriages between sons and daughters of Adam and Eve.
Does this sound like the sin of incest? In fact, such close intermarriage was not forbidden until the giving of the Law to Moses. And the Law protects people by preventing the selection of deleterious mutations that can be concentrated in the offspring of such close partnerships. But this world had only just gone wrong. In the second and third generations, and down to the time of Moses, there were not sufficient mutations for close intermarriages to be dangerous. So we have also answered the age old question: “Who was Cain’s wife?”