Why is there only one species of humans?

So, we can see new species being formed in a pretty short, recordable amount of time. Examples are seeing new bacteria species in labs, new bird species, etc. So, why only one species of human? Humans live in every habitat imaginable, with a wide range of required needs. Those humans have lived in those habitats... show more So, we can see new species being formed in a pretty short, recordable amount of time. Examples are seeing new bacteria species in labs, new bird species, etc. So, why only one species of human? Humans live in every habitat imaginable, with a wide range of required needs. Those humans have lived in those habitats long enough to have at least formed new species by now, so why haven't they? Why is it that after thousands of years, humans from the Sahara desert can still reproduce with humans from the arctic zones?
Update: Ok, so I like the answer by griblin, except I could add another question to that.... Humans are supposed to have evolved in about 5 million years. Apparantly, that is only enough time to get a new species, much less see the whole changes we do from humans to other primates. Why can that change occur so quickly?
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