Have you ever asked yourself where sin originated from? Did it start in heaven, or did it start on earth? Where did the first war take place? After Lucifer fell from God's grace and became evil incarnate, taking with him a third of the heavenly host, he began his seduction of Eve. And he is still at work today, tempting God's people to get them off their intended purpose - God's purpose - and get them into their will - their fallen will, and not God's will. Yes, the fallen one is still up to his old tactics.
God created a "seed-purposed people" who were to have children with one another by the commandment of the Lord. It was inbred in them to reproduce with their sisters and brothers, and this was very natural. It was very good, for God had commanded it so. The people on earth were without sin because these people had no covenant with God.
Years later, God decided to establish a covenant people, in order that his great love could be expressed on this earth, Adam and Eve. Adam was not the first man on earth, but the first covenant man on earth.