Adam and Eve were the only people on earth immediately after the six days’ work of re-creation. They were commanded to multiply and replenish the earth (Genesis 1:26-28), and they had sons and daughters (Genesis 4:1; Genesis 5:4; Genesis 6:1). The first marriages had to be between brothers and sisters in order to get the race started. Later, marrying close relatives was forbidden (Leviticus 18). Cain was over 100 years old when he took his wife and started his family in the land of Nod where enough people already lived to build a city (Genesis 4:16-26). Genesis 3:20 says Eve was “the mother of all living”-proof enough that she was responsible for both Cain and his wife.