/u/fruticosa explained here how genes end up being duplicated and suffer mutations throughout a timeline.Ricin comes from a unfortunate chain of events driven solely by evolutionary pressure and the randomness of nature.

The A chain of the protein, the one which inhibits the ribossomes, which causes it to effectively be toxic. It, among other similar ribossome-inhibiting proteins, has originated from a presumably defense-related protein that existed in an early angiosperm species.

It underwent it's own mutations over the ages, ended up being, in some of the higher plants, a gene that is duplicated several times and expressed differently in multiple phases of plant development, remember, not necessarily that means it has any function.

In some, however, it ended up becoming associated with another chain, the B chain. The B chain itself has originated from simple sugar-binding proteins, which basically any living being has in it's cytoplasm. Through the odds and evolutionary pressure, it ended up turning into a galactoside-binding chain.

That B chain, associated with A chain, is what is up with ricin. The galactoside-binding chain, once in our bodies, will bind with cell surface sugars, which therefore leads the cell to take the whole protein inside via endocytosis. Ribossomes are key parts of protein synthesis, their disruption by the A chain will lead to death in eukaryotic cells.

basically, two early proteins that were not really related at all went through numerous mutations, "fused" and originated it. At later evolutionary points, the dimer gene itself went through duplications and more modifications, coding for toxin-like proteins we see in some plants, but that's not the point.