It's as if the author tottally forgot that there are half a dozen people on the planet. I can just imagine Cain asking for a sign...
Cain: "But God? My punishment for murder is too great to carry. What if my immediate family suddenly forgot who I am and yet they also somehow remember that I was the first murderer? I'll need a sign for protection!"
God:"Oh shut up, you're lucky I don't acctually punish you, but now you want my protection? ... Well okay."
And then Cain was banished to... The Land of Fugitiveness?! Right, under ten people on the PLANET, and they already have a special land set aside for fugitives. What the heck kind of punishment is that anyways? They were already banished from Eden, so he was banished from the banished people. All he did was MOVE.
And then he built a city! In a few decades, maybe a century, Civilization progresses from cave people to urbanization.
Lore