How did different human races come about according to science?

Minor variations over time due to geographic location.

But, what we have termed as race does not have any real biological meaning.

"... if one were to walk up beside the Nile from Cairo, across the Tropic of Cancer to Khartoum in the Sudan and on to Nairobi, there would be no visible boundary between one people and another. The same thing would be true if one were to walk north from Cairo, through the Caucasus, and on up into Russia, eventually swinging west across the northern end of the Baltic Sea to Scandinavia..."

"Does Race Exist?" accessed 1/5/2012

http://www.pbs.org/wgbh/nova/evolution/does-race-exist.html