The Hebrew of Genesis 1:26-27 reveals God's creation of a specific man Adam, and also creation of the races of 'mankind'.
Hebrew 'aadam' with no article = mankind in general, the races.
Hebrew eth'ha'aadam = a specific man named Adam.
So when Cain was booted because of murdering his brother Abel, the people of the "land of Nod" where Cain took a wife from were already there, because they had already been created on the 6th Day like the man Adam was. The difference was that the man Adam was formed in God's Garden to dress it. But those of the land of Nod were outside God's Garden.
Translations of some of the Babylonian cuneiform tablets by Assyriologists revealed that Sargon, a Semite, showed up at ancient Sumer around 3800 B.C. and gave the Sumerians there knowledge of the sciences, and he built their first city. I believe that may have been Cain as Sargon I, and ancient Sumer being the "land of Nod".