Once upon a time there was a man by the name of Cain, who was a farmer, while his younger brother, Abel, tended livestock nearby.
Supposedly they were the sons of Adam and his partner Eve, who were the only 2 human beings on Earth after they were created.
God first made Adam but then saw that the man was lonely, so he took a rib from him while he was sleeping one night and used it to make Eve.
Eventually the boys came along and in due time had an argument as siblings do, so Cain picked up a club and killed his brother.
I'm guessing that while Mom, Dad, and their 2 sons had the entire planet to themselves, the killing came about because of a territorial dispute.
It seems that very little has changed....