Well, I read that all these vegetables were developed from a wild cabbage plant in England: Cabbage, Broccoli, Cauliflower, Brussel Sprouts and Collards. In fact, this was a correct answer on a question from the game Trivial Pursuit.
England's as good a place as any - makes sense. Anyway, collards too, they're all the same species. And you can crossbreed any or all of them pretty easily, and get some interesting hybrids.