incest

I got thrown out of Confirmation class when I asked about that, luckily my parents laughed and said I should keep asking questions.

Then we repeated it with Noah. The Bible is big on incest.
- The story about the arc is more modern than one might think:

And that's before Lot's daughters got him drunk so they could… prevent the extinction of the species.
- Yes daddy Lot

And even if the bible was wrong and they had sisters instead......
- I asked my Catechism teacher and she said
  “they did have sisters, they just weren’t documented because nobody cared back then” (misogyny in the Bible, who could have guessed?)
  “the inbreeding is why we’re no longer the image of God” (well damn) and “we didn’t have diversity before the Tower of Babel” (oh no, if only we didn’t dare challenge God, we could have prevented all those pesky different languages and cultures).
  Can you just admit that the Bible is full of nonsense rather than justifying it with downright batshit reasoning?

Forthermore, one of the brothers kills the other and is then Banished into the wilderness.
Perhaps this is the point where the theory of evolution and the Bible agree, and humans do indeed descend from apes.

Todd: Daddy, if Cain and Abel were Adam and Eve’s only children, how did they make more babies?
Rod: Did they make babies with their mother, or with each other?
Ned Flanders: Your mouth is hoping for a soaping boy. Now stop asking silly questions and go kill your brother!

Even if you don't accept that they parented all humanity, there's Noah's Ark, any diversity humanity had would mostly be reset at that point.