There was a man named Bartolome de las Casas who arrived in the New World in 1502 and saw firsthand the way that the indigenous population was treated. At the time, las Casas was not a priest, but he later joined the Dominican order in 1522. Las Casas denounced the abuses being committed against the indigenous peoples and gave several sermons arguing against their enslavement. He called for better treatment of the indigenous people and argued that enslavement did not encourage conversion to Catholicism and that native cultures could be used as part of spreading Catholicism. He lobbied for an end to indigenous enslavement, leading to the New Laws of 1542. Some believe that this is an example of las Casas's humanitarianism, but he likely was more interested in successfully converting indigenous peoples to Catholicism. Las Casas suggested that Africans were more biologically suited to be slaves and the general consensus is that Las Casas's protection of the New World's indigenous peoples and disregard for Africans set up the global slave trade.

http://www.lascasas.org/manissues.htm