After looking into this a bit further, it seems Neanderthals, Denisovans, other homo species (such as the Hobbits -- yes, that's a real thing), and modern humans are all descendants of Homo heidelbergensis. The different strains of Homo came about because as H. heidelbergensis left Africa, they went separate ways and each group developed separately from one another. But because they all developed from the same Homo strain, they could all interbreed with each other.
It's alot more complicated than that.
Also about the 'hobbits', they were much more primitive than other widely known hominids. Really more similar to Australopithecus (like ape-men) than any of the more developed humans.