Population of the Earth in the Early Times - Condensed Biblical Encyclopedia

Even in the days of Cain and Abel there were doubtless very many people whose names are not recorded. In the testimony of Moses we have this statement: "And the days of Adam after he had begotten Seth were eight hundred years: and he begat sons and daughters" ( Genesis 5:4 ). Other evidences are Cain's fear of death at the hands of his fellowmen, God's recognition of the ground of this fear and the means used to protect him, Cain had a wife ( Genesis 4:14-17 ).