Just my little joke, Nait. I understand where you're coming from, and I agree that having a simple backstory is much better than mucking around with separate planets/realities.
So how about this: Rather than have the races be separated by dimensions, simply have a pangaea-like world, in which a single species, separated by shifting continents, is split into five groups. Each faced with a new environment, and adapts accordingly, becoming the five races of our world.
