It is a curious thought that there may have been as few as four people on the earth when the first murder occurred. This story not only encompasses the first instance of a major human tragedy, but is likely also the etiology of both sibling rivalry, and as Robert Alter mentions in his commentary, the rivalry between herdsman and farmer (Abel was a herdsman and Cain a farmer.)
Additionally, it marks the second of several separations that occur due to disobedience. The first being the expulsion of Adam and Eve from the garden of Eden. Cain’s story involves leaving home (and possibly his parents and other siblings) to become a wanderer, though he eventually settles elsewhere and built a city. The third such story occurs on a much larger scale at the tower of Babel, where God confused the language of the people so they could not understand one another. This was done for several reasons, but one of them being that after the flood, God commanded the people to populate the earth, with the implication that they were to spread out rather than remain congregated in a localized area. Thus the story of Cain and Abel is significant for several reasons.
I hope you enjoy my notes, and as always, feel free to leave a comment with any questions you have, or if you would like anything explained further.