I find it strange that this topic has had so many replies and covered so many different aspects of "race". The original poster simply asked how different human races originated.
Firstly a human "race" is poorly defined and probably has no strict scientific definition. But in generally spoken English, it is common to speak of "races" such as the Australian Aboriginal race etc. I suppose in these general terms a race is a group of people who have certain characteristics which are different from other groups of people. Early European settlers in Australia certainly recognised that the Aborigines were different - a different "race".
They have evolved to some extent to adapt better to their isolated environment. There are clearly genetic differences between Aboriginals and European settlers. Aborigines cannot digest certain food groups, but seem immune to certain diseases which are prevalent among Europeans. The Aborigines have had about 50,000 years to adapt to their environment.
So in summary I believe a "race" is a local variation of the same overall human species. The variations are caused by adaptations to local environments. But we all belong to the one species Homo sapiens.
That is not to say one race is better or worse than another.
Due to mixing of races it is often not possible to look at one randomly chosen person and say what "race" he is. But it is possible to investigate "pure" or "near pure" members of a "race" (isolated group) and it will normally be found that they have charactersitics in terms of genetics, language, culture etc which set them apart from surrounding races or groups. Genetic scientists aim to investigate such "near pure " groups to try to unravel the history of mankind.
The Kalahari bushmen, pygmies, Australian Aborigines, certain Bedouin tribes are examples of such races or isolated groups.
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