[shekkian]

How is it that a single human ancestor 2 million years ago gradually populate the Earth and yet not evolve into a dozen different species?

[RoughDobermann]

How is it that a single human ancestor 2 million years ago gradually populate the Earth and yet not evolve into a dozen different species?

How do you know that a dozen different species will not evolve. Who knows what will happen over the next five million years?

[jennyp]

How is it that a single human ancestor 2 million years ago gradually populate the Earth and yet not evolve into a dozen different species?

Too much miscegenation goin' on all that time kept the races "impure". This prevented them from drifting away into true species.