The Atlantic Slave trade developed primarily because of the explosion in demand for sugar cane. Sugar had been unknown in Europe prior to the Crusades; foods were sweetened either with honey or fruit juice. The introduction of cane sugar resulted in an insatiable demand, which made sugar cane production exceptionally profitable. The islands of the Caribbean and Brazil were well suited for sugar cane production, and large plantations for its cultivation sprang up in those areas. Large scale production of sugar was necessarily labor intensive and created a demand for labor, preferably free labor. Originally, American Indians were used as slave labor, but hey proved unreliable for a number of reasons. Thereafter, the Spanish began importing African slaves to work on the sugar plantations. So prolific was this practice that by the time the first Africans landed at Jamestown in 1619, over one million African slaves were working on Caribbean sugar plantations. Nations which did not engage in colonization, including Denmark and Prussia, chartered joint stock companies to deliver slaves to the Caribbean at immense profits. Thus it was the sugar revolution which gave rise to the Atlantic slave trade.
The Atlantic slave trade was not something that arose overnight in the form that became famous. Instead, it developed gradually over a number of years.
The trade can be said to have started with the Portuguese. Their explorers made their way down the coast of Africa in the 1400s. They made contact with Arab merchants who procured slaves for them from the sub-Saharan kingdoms. These slaves were taken in relatively small numbers and were used for domestic service in Europe and for some agriculture on islands off Africa and Europe. Eventually, the Europeans started to get their slaves directly from West Africa as their trading routes expanded and as they came to need more slaves to work in the Americas. This slave trading was done mainly by Spaniards at first as the pope had given them the exclusive right to trade with the New World (this was before the Reformation so everyone essentially obeyed).
At this point, the Europeans started systematically trading for slaves. They built trading posts where they kept slaves that they had bought until ships could take them away. The Europeans traded with coastal kingdoms for slaves that those kingdoms’ people took from other states and kingdoms. This form of the slave trade was essentially the final stage. The only changes that occurred were in the identity of the traders. The English came to dominate the trade in the late 1600s and beyond.