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.
Adolph Bougereau, The First Mourning (1888), emphasizes Adam and Eve's despair over losing their boy, who is depicted as feminine, almost pre-Raphaelite.
In his 1899 depiction, Danish painter Laszlo Hegedus gives Abel (facing us) a self-righteous sneer, while Cain (the one with the bare backside) has red hair, symbolising anger.
John P. Reilly (1928-2010) was a Catholic artist who drew inspiration from the Byzantine world. There's no nudity in his Cain (1958), but I thought it was interesting that Abel, the good guy, is tall, thin, and handsome, while Cain is a short, squat, grunting Morlock. He's also trapped; maybe his genes have already doomed him to be a murderer.
Marc Chagal, the French-Jewish primitivist, gives us a penis in his 1960 version.
The Bible doesn't say how old Cain and Abel were, but most artists make Cain middle-aged and Abel a sallow youth. Eric de Saussure's 1968 version makes them both boys. Oddly, Cain is dark-skinned and Abel light-skinned.
Maurice Heerdink makes them both teenage in his ultra-realistic, brightly-lit, homoerotic version (2001).
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.