system
A chat on subreddit /r/NoStupidQuestions
human
Genesis 5:4 - After Seth was born, Adam lived 800 years and had other sons and daughters.

Incest. The answer is incest.
gpt
Someone was doing a genetic study and I don't have a link but it basically looked like incest/close cousins unless you went for another race and even then it was pretty iffy. 

So yes, lots and lots of incest.