Nonsence! Bemused have the right answer! Please go back and read Genessis. You will find that Cain argued with God that "other people" might want to kill him, proving that there was indeed other humans on the planet. We need to realize that the Bible is an account on the life of the Hebrew people an not of the whole human race. Also! Please remember that the first 5 books of the bible where written by Moses, who was an educated man, but who found himself in the unique predicament of explaining to 4 million uneducated slaves, the existence of a magnificent, almighty sovereign creator of the universe. Had he done it in a philisophical, cientific or educated manner, NOT one of them would have understood. The Bible is full of misteries and unimaginable treasures, but we must be carefull on trying to interpret it, specially if you choose take everything literally.
There were other tribes of people wandering around at the time. If you took Genesis completely literally, it wouldn't make any sense. Clearly, the human race wouldn't have progressed if Adam and Eve were the only two people on earth to have the first two babies.
Adam and Eve had more than just those two children. Cain also had a brother whom he killed. Other than that though, later on in the Bible it says that Adam and Eve had more children. He had to marry one of his sisters for life on Earth to continue.
Everything started with Adam and Eve, so Cain's wife would have to have been his sister. Unthinkable now, but unavoidable at the time. Not long after humanity began to increase in number, however, this was forbidden.
Where did Cain get his wife if there was just one family? Gen. 3:20: “Adam called his wife’s name Eve, because she had to become the mother of everyone living.” (So all humans were to be the offspring of Adam and Eve.) ( rest of answer in comments)
Adam and Eve was a creation story. Number one, people wanted to think they understood how everything got started. Number two, our brains are hardwired to understand stories with plots and recognizable characters much more easily than facts with statistics. Numbers. Evidence.
In Genesis 5:4 we read that Adam and Eve had other children including females besides Cain and Abel: "And the days of Adam after his fathering Seth came to be 800 years. Meanwhile he became father to sons and daughters." From considering the entire Bible account we can see that Cain found his wife from his own family. His wife was one of his fleshly sisters, a daughter of Adam and Eve. The Bible makes it clear at Genesis 3:20 that Eve was “the mother of everyone living." So the theory that Cain found his wife from some other family on Earth is not in harmony with the Bible.
Although Adam and Eve sinned and became imperfect, Cain and his family were still so near to physical perfection that the children they produced did not suffer the same bad effects as children born of such unions today. Even 2,000 years after, faithful Abraham married his half-sister Sarah, and God did not disapprove. (Gen. 20:12) It was yet another 450 years or so before God saw fit to provide a body of laws that forbade incest on penalty of death. (Lev. 18:8-17) By that time imperfection had apparently developed to such an extent that no longer was it safe for close relatives to marry. Hope this helps!
Once God completed making Adam & Eve, what was to stop Him from then putting lots of other people in various place in the world before Cain ever grew up big enough to get married? Perhaps while Cain was still a baby, God may have put people in the land of Nod.