What is the connection between slavery or racism

The relationship between slavery and racism is no doubt undisputed. However, there is a deeper, "chicken and egg" question there: Was African slavery the cause of racial prejudice; or did a preexisting prejudice against those of African descent lead to their enslavement? I wrote a rather lengthy paper on this topic several years past and concluded (safely, I thought) that racial prejudice preexisted. I received a rather long email from a distinguished scholar who respectfully disagreed and cited considerable evidence to that effect. This is an issue that perhaps has no right/wrong answer; but I would be curious to hear the ideas of others who may have researched the topic.