(I dont remember the exact to qoute so corecct me if im wrong.)
In the bible cain kills abel and gets banished. God gives him a mark so that no other humans can kill him.
What other humans? Adam and eve gave birth to cain and abel and other than that there were no other humans right?
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There is no text that states specifically that Adam and Eve were the only people. It only states that they were the first.