Racism is not a natural state for humans. It has been created to defend the practice of slavery, and very consciously.
I found this out while reading the first chapters of Howard Zinn’s “Peoples history of the US”.
Also Michael Parenti has a very nice lecture on the historic causes of racism http://www.radio4all.net/proginfo.php?id=1848 called “Racism and the Ideology of Slavery”. Please download it and listen to him, it is fascinating.