Consider the following excerpt from the trial record as Darrow interrogates Bryan:
Q - Did you ever discover where Cain got his wife?
A - No, sir; I leave the agnostics to hunt for her.
Q - You have never found out?
A - I have never tried to find.
Q - You have never tried to find?
A - No.
Q - The Bible says he got one doesn’t it? Were there other people on the earth at that time?
A - I cannot say.
Q - You cannot say. Did that ever enter your consideration?
A - Never bothered me.
Q - There were no others recorded, but Cain got a wife.
A - That is what the Bible says.
Q - Where she came from you do not know?
As you well know, Cain killed Abel! So I thought it was just the three of them left...Adam, Eve & Cain. No?