If the people who purported to be such great believers were not such wetbrained morons so often; this question would be an easy softball:
“And Cain went out from the presence of the Lord, and dwelt in the land of Nod, on the east of Eden. And Cain knew his wife; and she conceived, and bore Enoch: and he built a city, and he called the name of the city, after the name of his son, Enoch.”
The story of Adam and Eve is always mis-represented as the story of all humanity; which could not be further from the truth. It is the story of the first followers of Judaism. The origin of “God’s Chosen People”. It stands to reason to think that a creation story about “the chosen people” would leave out the existence of people who live “just east of here” they weren’t chosen. Only Adam and Eve were.
The next time someone says “It’s Adam and Eve not Adam and Steve” you can reply “Sure, but it’s also Eve and Cain and Eve and Cains Son and all of Cain’s brothers too, right?” Then when they are freaked out you can point them to the passage above and say:
Look dummy, the Bible isn’t a literal telling of the origin of life on earth. If anything the old testament is a Gentile-Friendly version of the Talmud minus the heavy thinking and the emphasis on waiting for the Jesus to come. It’s a set of stories the describe the struggle of the Jewish People to live up to their Deity.