The trans-Atlantic slave trade was the largest long-distance coerced movement of people in history and, prior to the mid-nineteenth century, formed the major demographic well-spring for the re-peopling of the Americas following the collapse of the Amerindian population. Cumulatively, as late as 1820, nearly four Africans had crossed the Atlantic for every European, and, given the differences in the sex ratios between European and African migrant streams, about four out of every five females that traversed the Atlantic were from Africa. From the late fifteenth century, the Atlantic Ocean, once a formidable barrier that prevented regular interaction between those peoples inhabiting the four continents it touched, became a commercial highway that integrated the histories of Africa, Europe, and the Americas for the first time. As the above figures suggest, slavery and the slave trade were the linchpins of this process. With the decline of the Amerindian population, labor from Africa formed the basis of the exploitation of the gold and agricultural resources of the export sectors of the Americas, with sugar plantations absorbing well over two thirds of slaves carried across the Atlantic by the major European and Euro-American powers. For several centuries slaves were the most important reason for contact between Europeans and Africans.
What can explain this extraordinary migration, organized initially on a continent where the institution of slavery had declined or totally disappeared in the centuries prior to Columbian contact, and where, even when it had existed, slavery had never been confined to one group of people? To pose the question differently, why slavery, and why were the slaves carried across the Atlantic exclusively African? The short answer to the first of these two questions is that European expansion to the Americas was to mainly tropical and semi-tropical areas.