Consider this picture:
Ask yourself, how come we are able to tell which geographic region someone's ancestors come from, just by look at someone's face? And even more so when looking at someone's DNA.
Why is this so?
EVEN if you believe in the adam and eve story... then still this would require an explanation.
The answer is simple: genetic isolation.
At some point, groups of humans migrated away from the "mother" population. They settled in various parts of the world. For thousands of years, they wouldn't intermix - or at least very very little.
So we had these "islands" of groups of people which were all pretty much genetically isolated. With perhaps minor genetic exchange with directly neighbouring islands.
This is what evolution does.
Humans in the original "mother" population(*) surely are were pretty much the same "race". Once groups migrated away and genetically isolated themselves from the rest, they diverted away. Evolving into caucasian and asians.
Now, with globalization and vastly increased international travel, all these genes are quickly thrown into a big jar again. But had that period of genetic isolation continued, then speciation of all these groups into subspecies of homo sapiens would eventually become inevitable. That, or a dead end (=extinction of the group).
(*) fun fact concerning this mother population and a specific piece of evidence to support this idea. If you have a single mother population, then that population holds all the genetic variation. If a group splits of and migrates away while the majority stay put, then for the lineage of that migrated group, this represents a genetic bottleneck. As most of the variation stays behind and only a subsection moves away.
Now, take the most "pure" caucasian and most "pure" asian you can possibly find. With "pure" I mean someone who's ancestors for generations have been locals. Take a DNA sample from both and compare it. There will be LESS genetic variation between these two samples, then there will be between two random unrelated africans who live in the same city in africa.
Ain't that funny?