Theological, Non-Essential thought

So, God creates man.  Adam and Eve are created.  They have two boys, Cain and Abel.  Cain kills Abel.  God takes pity on Cain, and says whoever kills him will have sevenfold vengeance on them. God puts a mark on Cain so people will know not to kill him.  So, hold on a minute.  Other people.  His family knows him.  So, are there other humans on the earth?  Who does he need to be protected from?  Did God create more humans after Adam and Eve, but it just is not recorded.  So the population of the earth had to be somewhat substantial for people to not know Cain, but to know his mark.  Crazy.  What are your thoughts?

SNYDER