BOTR said-
I was always outraged by Jehovah's rejection of Cain's sacrifice. What is wrong with agriculture? I see nothing wrong. In fact, it seems a step up from hunting.
Except Abel wasn't a hunter, but a shepherd: he raised domesticated animals (the account doesn't state what kind of flock, or what he did with them, since they couldn't have been raised for their meat since it wasn't yet added to the menu).
Supposedly Cain had his 'green thumbs' completely nullified (unlike Adam, who had the power of his green thumbs diminished, not completely nullified), which left him with a slight problem: since animal flesh wasn't yet added to the God-approved diet, and he couldn't grow any food, what was Cain to do to obtain food?
The story says Cain established a city (!), presumably depending upon trade with those who hadn't been cursed, and that's the Genesis explanation for the move towards urban centers and the development of high-tech things like musical instruments, culture, metal-working to create tools, etc.
Adam