Why was African slave trade necessary?

Slavery has been around since the beginning of human history. Slaves were prisoners of war, convicts, or peoples that were considered inferior. Europeans considered Africans to be inferior in a"Social Darwinism" way. But before Darwin even appeared,Europeans considered themselves superior to Africans because of their advanced technology and civillization. It was actually rival slave tribes thatwould capturethe enemy tribe members and sell them to the Europeans. The African slaves tribes really hit their stride in the early 1800's when there began to heavydemand for slaves in the newfrontiers such as the US. Big Plantations in the South America, Carribean, and in the south US, neccessitated a large manlabor pool and the slaves were necessary in order for them to make the agarian culture thrive at its highest level.