There is no arguing that bigotry in myriad forms existed since the beginning of humanity. However the discussion of classification by race may be traced around the time of Carl von Linneaus in 1735.

Linneaus was a Swedish taxonomist that proposed five taxa: Africanus, Americanus, Asiaticus, Europeanus and Monstrosus. They were based on place of origin at first, and later on skin colour. Each race had certain characteristics that he considered endemic to individuals belonging to it. Native Americans were choleric, red, straightforward, eager and combative. Africans were phlegmatic, black, slow, relaxed and negligent. Asians were melancholic, yellow, inflexible, severe and avaricious. Europeans were sanguine and pale, muscular, swift, clever and inventive.

Classical civilizations from Rome to China tended to invest much more importance in familial or tribal affiliation than with one's physical appearance. In many ancient civilizations, individuals with widely varying physical appearances became full members of a society by growing up within that society or by adopting that society's cultural norms (Snowden 1983; Lewis 1990).

Not until the 16th century did the word "race" enter into the English language. Meanings of the term in the 16th century included "wines with a characteristic flavour", "people with common occupation", and "generation". A meaning of "tribe" or "nation" emerged in the 17th century. The modern meaning, "one of the major divisions of mankind", dates to the late 18th century, but it never became exclusive (note the continued use of the expression "the human race").
http://en.wikipedia.org/wiki/Race_(classification_of_human_beings)

I became interested in the link between slavery and racism when reading an article on Irish Slaves which quoted Jacques Chirac:
children should be taught about slavery at primary and secondary school as part of the national curriculum. "Slavery fed racism," he said. "When people tried to justify the unjustifiable, that was when the first racist theories were elaborated."

Some discussing Jacques Chirac's comments, wondered whether it should be the other way around, but in fact it turns out he is at least partially correct.

Slavery dates back to the Code of Hammurabi in 1760 BC, was used by the Egyptians, Chinese, Tibetans, Greeks, Romans, and almost every civilization. Over 10% of England’s population entered in the Domesday Book in 1086 were slaves.
http://en.wikipedia.org/wiki/Slavery