It's not in "the book" but god pulled off another creation in the next valley and that's where these wives came from. They have their own "book". And ever since humans have been coveting whatever the people in the next valley have. Either game, gathering food, material resources or the women. It's still going on today but at a much larger scale
@Matias Hey, it's as good a fiction as theirs
Cain and Eve hooked up, hence the word for it, MF's.