I yield to your expertise, but my brief bit of web research before I posted it seemed to show overlap between the upper range of the halfing size range and the lower range of some pygmy groups. At any rate if you allow that homo floresiensis was a human then it's clear that the homininae size range allows for pretty much all the PC races.
In fact drawning in extinct homininae like Homo floresiensis and neandertalensis and you can cover pretty much any non-giant humanoid. Even some giants depending on where you come down on gigantopithecus.
I just finished reading
The Forest People, which is about pygmies. It mentions the size myth, in fact - while they are a short people, they are not as short as D&D halfligns, which is what got my feathers all ruffled up.
As for floresiensis, one of the problems with this particular field of anthropology is that everytime someone makes a new discovery, he tries to label it as a new species of hominid. Which is what floresiensis is - during my time in classes (about three years ago, now? Maybe a year more), it wasn't even considered a homonid by mainstream anthropologists for this reason. Looking on the web, I see that opinion is changing, for a variety of reasons.
But, if you look a bit closely, you'll see that chimps are also members of the hominid group. In other words, I wouldn't read much into it. So, I wouldn't allow it as a "human" anymore than I'd say a chimp was a "human" (and that's not insulting chimps, by the way... I love those little guys!)
Gigantopithecus is AWESOME, by the way, but it's not a species of hominid, and wasn't ever considered to be. From what I've read (and I could be wrong, it's been a while), it probably had the intellect of a dumb gorilla. But, damn, it's a cool species!
As a sidenote, that's only sort of related: Neanderthals are an interesting case, because a lot of anthropologists (and myself) believe that they are really just a variant homo sapiens, and that they didn't "Disappear" or go extinct, but instead interbred with mainstream homo sapiens. There are a few pieces of evidence to support this - here's my favourite.
Feel the back of your head, where the neck meets the skull. Some people (primarily those of northern european descent) have a small protrusion there, a sort of "bump" (I have one). Archaic homo sapiens (those skeletons found that first show all the signs of homo sapiens... but they look super weird) do not have these. Neanderthals do. They say having one is evidence that there's a neanderthal somewhere in your family tree.
I always thought that was neat.
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Anyways, that's the science of the situation. But, I'll be the first to say "to hell with science" in a fantasy game. There is no reason why you can't have three-foot tall humans in a fantasy game. Obviously. While I may not believe it's ever happened on Earth, I could be wrong - anthropology is a changing field.