In the scale of things, 400,000 years isn't really any time at all. I'd note that the most rapid development occured relatively recently, before that length of time people lived relatively similar ways of life reducing the need for adaptation.
Why fix what isn't broken :P
I'm not exactly the most knowledgeable about this kind of thing, so I can't exactly spout off what meagre knowledge I've gleaned off textbooks and the odd documentary or two. Only other thing that comes to mind that might be relevant are the changes and ways people's body's have adapted to correspond to the areas in which they live. Not exactly enough to constitute a different 'species' but enough to make a difference all the same. Hair type/ skin-color/ body-structure development etc. that a particular people develops to adapt to the environments they find themselves in.
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