There is more to support your idea that there was already a population of humans on earth then you realize. When Cain kills Abel (out of jealousy because Lord God preferred Abels offering to his) Yahweh sends Cain out into the world to be a drifter. Cain fears that he will be killed so Yahweh puts a mark on him to prevent it. But if Adam and Eve were really the first people, and at that point they had two sons- maybe three I can't remember if Seth was in the picture yet- who would Cain be afraid of unless there was already a population of people? That was a very early confusing question of mine that the church never answered properly.