The Muslims called these Africans Zanj, and they were a distinct and despised group, distinguished from other North Africans by the color of their skin. The Zanjan territory held by the Abbasids staged one of the first big slave revolts in 869 CE, and it may be that this revolt was so devastating that it convinced the Abbasids that large-scale plantation-style agriculture on the Roman model just wasn't worth it. But by then they'd connected the Aristotelian idea that some people are just naturally slaves with the appearance of sub-Saharan Africans. The Spanish and the Portuguese, you no doubt remember, were the Europeans with the closest ties to the Muslim world because there were Muslims living on the Iberian Peninsula until 1492, so it makes sense that the Iberians would be the first to absorb these racist attitude toward blacks. And as the first colonizers of the Americas and the dominant importers of slaves, the Portuguese and the Spanish helped define the attitudes that characterized Atlantic slavery, beliefs they'd inherited from a complicated nexus of all the slaveholders who came before them. Atlantic slavery was a monstrous tragedy, but it was a tragedy in which the whole world participated. And it was the culmination of millennia of imagining the other as inherently lesser.