system
A chat on subreddit /r/NoStupidQuestions
human
It is uncertain, there seems to be at least two possible interpretations. First, Adam and Eve had more than Cain and Abel. 

&gt; After Seth was born, Adam lived 800 years and had other sons and daughters. (Genesis 5:4, New International Version).

So, the answer could be that incest was the solution. It certainly seems to be the solution later in Genesis after the great flood, so it could apply here as well.

Another possibility is that God created other people.

&gt; So Cain went out from the Lord’s presence and lived in the land of Nod, east of Eden. Cain made love to his wife, and she became pregnant and gave birth to Enoch. (Genesis 4:16-17, New International Version).

Who was Cain's wife? Where there other people in Nod? So, maybe God made others.
gpt
Probably the best answer you will get here.