They are the children of Adam and Eve, the first two brothers in the world. They both offer food to God, Abel animal meat and Cain fruit.
God, being a carnivore, prefers Abel. Cain gets jealous, and in a fit of rage, kills his brother.
He is then forced to wander, but he worries that everyone he meets will want to kill him (the world has filled up quickly). So God gives him "The Mark of Cain" so he will be safe.
Rather thoughtful. I would have gone with life in prison for murder, but...
The story has a number of plot holes and inconsistencies. But look at those sculpted abs and enormous biceps!
Throughout history, artists who wanted to depict the homoeroticism of two muscular men together, without women around, have drawn on Cain and Abel. They struggle, strain, press together so tightly that you can almost forget that they're trying to kill each other.
And, in the modern era, you can comment on warfare, bigotry, and homophobic hate crimes.
Abel is the quiet, gentle, gay-coded shepherd. Cain is depicted as a big bully, a rough-and-tumble farmer.
I have to include this version by Bill Hoope (2001), if only because I want to know where they got the globe, and why they're attacking it with animal bones.