Right now human = homo sapiens. There are no Neanderthals running around, or Cro-Magnons or Homo Erectuses (Homo Erecti? And stop giggling). However, it wasn’t always so. Separate human species have not just emerged through sequential evolution but have lived side-by-side.
It’s quite hard to imagine, but not so long ago we shared Europe with Neanderthals for a prolonged period. On one island or other (I shall endeavour to remember where) people lived alongside a hobbit-like race of rather diminutive humans. These other people were apparently exterminated after causing serious harm to the local homo sapiens, who took rather serious and understandable offence.
There’s a region in South Africa that’s very secluded, and for a long time a group of homo sapiens there slowly began to evolve into a separate species (or perhaps subspecies). The key difference was language, with this isolated race developing a bird-like method of speaking utterly unlike the divergent but fundamentally similar languages with which we’re all familiar.
Could we, if we discovered them in a rain forest or cave system, live peacefully alongside another race of humans? I find it impossible to believe, sadly, given that we can’t even tolerate the same species of humans very well. It is, however, an intriguing thought.
Differing human species or subspecies also present fantasy writers with an alternative to the popular but not especially innovative standard of having humans, elegant elves and dumpy dwarves co-existing. Although modern humans have some racial differences (Chinese people have single eyelids, for example) we don’t have multiple species today, but it’s not too hard to imagine.
So, what differences could be used, and what differences existed between real world human species?
I remember watching a programme (Horizon, I think) about Neanderthals, which was fascinating and described in some detail the appearance, voice and other features of the extinct race. For a start, they had much longer rib cages, and didn’t have a waist. This made them sturdy but inflexible. In addition, their inner ear was different to ours, so they had inferior balance. Their voice would be nasal and high-pitched and I think that they would have better endurance/strength than us.
The hobbit race I mentioned above were, obviously, much smaller than us. In addition, they had a rather nasty habit of stealing things, which did not endear them to their neighbours.
A fundamental difference is whether a species is exclusively bipedal (like gibbons, or us) or can pick and choose (like gorillas or chimps). If a species were bipedal and quadrupedal then its eyesight might be worse and sense of smell better, and its arms and legs would probably have to be equal in length.
The three human species I use in the still untitled (I’m thinking of Bane of Souls, perhaps) book are quite similar. The Felarians and Dennish are almost identical, save that the former have darker skin, but the Kuhrisch are taller, stronger and somewhat resistant to the cold (but, unlike the other two races, they’re almost never magically gifted).