What is the nature of the disagreement between historians in explaining the enslavement of African peoples?

The main area of disagreement, between historians throughout the ages, has been the underlying cause that brought about slavery of African Americans. Since the 1950's and continuing even till today two major theories have abounded. One theory, we all seem to know, is that this form of slavery was based on racism and the belief that one peoples were superior to another. The second theory was that slavery was brought about by economic conditions, and that this form of slavery was the most cost effective way of managing large farms that had little profit. In 1996 robin Blackburn united the theories in the book "The Making of New World Slavery." Debate over this topic still continues though.

What the above areas of disagreement fails to do is approach the fact that Islamic slave traders had been shipping slaves from West Africa to areas in the Middle East before the Portuguese and Dutch slave ship "industry" began.