Where did the people of Nod come from?

I have watched your program for a long time and I believe you are one of the most learned of all. I have a friend that is an Atheist and he asked me this question. Adam and Eve had Cain and Able and Cain killed Able. He wandered to the land of Nod and took a wife. Now the only people on earth, as far as I understand is Adam, Eve and Cain. Now where did the people of Nod come from?  If you could help me on this I would really thank you so much. 

Cain, Able and Seth are the only children of Adam and Eve which are named in the Bible.  But Genesis 5:4 says:  “And the days of Adam after he had begotten Seth were eight hundred years: and he begat sons and daughters.”  In that time there were hundreds born and their children were having children and so on.  There could have been a thousand or more by the time Cain was exiled.  He himself had a wife already even though no child of his is mentioned until Enoch.  Genesis 4:3 stated:  “And in process of time it came to pass, …”  Apparently much time had passed and some of those descendants migrated to the land of Nod.