Why was the mark of Cain necessary? 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.
First, it doesn't necessarily say that Cain was the first born, only the first who did anything significant enough to be included in the record.

Second, even if Cain was the first son, that doesn't preclude Adam being the one to hypothetically take vengeance.  For example, The Law of Moses included a passage requiring parents of incorrigible adult children to be the ones to bring the accusation before the elders of the city.  (Deuteronomy 21: 18-21.  And as shocking as this may sound, consider the psychological effect: it makes an amazing motivator for parents to make sure their kids turn out well!)

Also, when you consider that Genesis tells of people living for centuries, there's no reason why someone born after Cain wouldn't have grown to adulthood, then run across him, recognized or found out who he was, and tried to take vengeance into their own hands.