One of his sisters..and had a son, Enoch:
Genesis says that Adam and Eve had two sons - Cain and Abel. Abel was killed by Cain after some dispute. Cain then married a woman, whose identity has not been revealed in the Old Testament.
4:16 And Cain went out from the presence of the LORD, and dwelt in the land of Nod, on the east of Eden. 4:17 And Cain knew his wife; and she conceived, and bare Enoch: and he builded a city, and called the name of the city, after the name of his son, Enoch.
Who was Mrs. Cain? It can only be assumed that she was Cain's sister! If it wasn't a sister, then it must have been his mother. If it was not his mother or his sister, then she did not evolve from Adam and Eve. This contradicts the original assumption that EVERYONE came from Mr. & Mrs. Adam.