I believe there is
one species of humans and we can/could all interbreed, we are Neanderthals and any differences in humans today (as in the past) are due to regional variations.
Modern humans are differentiated from Neanderthals, Homo-Erectus and other 'archaic' humans by physical measurements of their bodily features.
Now, if we say that everyone alive today is a
Modern Human, that presents a problem. If we measure the features of all humans alive on the earth today, the range of features easily include Neanderthals, Homo-Erectus and even some 'Archaic' humans...The differences are regional.
Some of the Australian Aboriginies have brow ridges that are more 'severe' that the Neanderthals, yet we call them moderns but, not so Neanderthals.
I think Wolpoff, Sharpe and Hu (the Multiregionalist) have it correct, we are all one with regional differences
Journey Of Mankind (Modern Humans)
Neanderthals aren't on that map, because we (homo sapiens) are not genetically related to Neanderthals (homo neanderthalensis) -- we both split off from the same family tree but the branches are separate.