.The Bible records the fact of Adam and Eve leaving the Garden of Eden in Genesis 3:23,24, (NIV) "So the Lord God banished him from the Garden of Eden to work the ground from which he had been taken. After he drove the man out, he placed on the east side of the Garden of Eden cherubim and a flaming sword flashing back and forth to guard the way to the tree of life." This was before Adam and Eve had any children.
Then in Genesis 4:1 (NIV) the Bible records, "Adam lay with his wife Eve, and she conceived and gave birth to Cain." In Genesis 5:4 (NIV) it states, "After Seth was born, Adam lived 800 years and had other sons and daughters."
Genesis 4:17 (NIV) mentions Cain and his wife having a son named Enoch. Obviously, the earliest inhabitants of earth had no other choice than to marry their brothers and sisters in order to fulfill the divine command, "Be fruitful and multiply." The custom long remained in vogue as seen in Abraham's marriage to his half sister Sarah. Such marriages were later prohibited (see Leviticus 18:6-17).