Adam and Eve were not human in the sense that they were created by G-d in order to become the first couple. Abel was killed by his brother Cain. The question is who was Cain's wife? Probably a daughter of Eve. Was this incest? It probably was not considered to be a forbidden act then, since the Law was given much later and the dominant principle imposed by G-d was "be fruitful and multiply,...", overruling opposing considerations. Also the first ten generations were biologically much superior to the later generations as is evident from the fact that they lived much longer, showing that incest did not produce harmful genetic defects then.
