Why does the Bible say Adam and Eve were the first humans? The Bible clearly states Cain was sent to a land called Nod. If there was no people there, then why was he afraid someone would hurt him?

Why does the Bible say Adam and Eve were the first humans? The Bible clearly states Cain was sent to a land called Nod. If there was no people there, then why was he afraid someone would hurt him? When you read the Genesis account you have to remember that it was written by Moses 2500 plus years later, so when he says things like, Noah took 7 each of the clean animals and 2 each of the unclean animals on the Ark, these clean and unclean designations were not known to Noah but we're given to Israel in the Law given Israel through Moses, so Moses used those term the people currently understood.This is no doubt why Moses could say that Cain was exiled to the land of Nod. That name for the area where Cain went was unknown in Cain’s time but was known to Moses and others in his day.