The Atlantic slave trade had its origins in the “discovery” and conquest of the New World.
As the New World was settled by Europeans, they found that they needed more labor at lower prices than other Europeans were willing to give. They also found that it was too hard to get the Native Americans to work (because they could escape or because they died from disease). Therefore, they needed another source of cheap labor.
Europeans had been in contact with Africa long before the discovery of the New World. They also knew of the slave trading that went on in parts of that continent. Because of this, they started to buy slaves from African rulers and to transport them across the Atlantic to the New World. They would buy slaves from coastal rulers who would kidnap them from areas farther inland. This combination of the European need for cheap labor and the African rulers’ desire for wealth and power caused the Atlantic slave trade to develop.