The Cain and Abel story goes back to an earlier(like 2000 years earlier) Sumerian story. In the Sumerian story, the farmer(Cain) is preferred by the gods for his vegetables and grain offering over the shepherd's(Abel) offering of meat. The Sumerians were mostly farmers, and so the gods were seen as favoring farmers. When the Hebrews got the story, they turned it around and had God favoring Abel, which enraged Cain so he slew his brother Abel. The Hebrews were mountain herders and played fast and loose with the older stories, like the flood, and put a moral bias in which wasn't in the original Sumerian stories. To read the originals check out the work of Zechariah Sitchin. The books are easy to find in paperbacks.