I think the key is Eve herself. God created her to be adams " Mate", not his sister, so it was not incest. Yet if one takes the view of incest, then their view is off base from the get go. God created her a " Seperate individual, then he " Married them himself" in Gen.2:24, and declared Eve to be his " Wife." Why he used a rib from Adam to create her, I am not quite sure, I think it has more to do with symbolic unity, rather than some perverted incest.
I think Eve is the key here, God must have given her womb the ability to give birth to different races of humans, different genes and different tribes had to have come from her, in order to avoid the obvious assumption of incest.
But still, I am not really sure, its more of a mystery to me, we just don't really know.
Peace.