In Genesis 4:14-15 (NRSV) it says:
14 I shall be a fugitive and a wanderer on the earth, and anyone who meets me may kill me.’ 15 Then the Lord said to him, ‘Not so! Whoever kills Cain will suffer a sevenfold vengeance.’ And the Lord put a mark on Cain
Who could possibly kill Cain if Adam and Eve are the first people?
Cain was not married at that time (suggesting he was young) and was the first born there doesn't seem to be a rational explanation that someone that was born of Adam could have grown up without knowing Cain and attack him while he was wandering.