This question excites me because I'm actually studying African History. I'm on my phone so I will try to give a VERY abbreviated version of how the Trans Atlantic Slave Trade came to be....

As most point out the slave trade has always existed across the globe regardless of race. Most were war captives and used in domestic tasks and at times on large work projects for a state. As for the Arab connection with the slave trade, a trans-Saharan trade had existed for quite some time (I can't give a precise date, but centuries before The Portuguese arrived off the coast) where caravans would cross the Sahara and go south to trade with subsaharan Africa along the Sahel region. (look up the famous Timbuktu - it was one of these depots at one period). Keep in mind, this was by no means a trade exclusive to enslaved people, in fact there were all sorts of good traded. Of the slaves that were traded, they would have been war captives a vast majority of the time, and in a drastically smaller quantity than what came with the trans-Atlantic trade. Keep in mind, the slave trade here was developing same as the rest of the world in quantity as well as remaining largely color blind.

Fast forward to the end of the 15th century. The age of discovery is upon the world, and the Portuguese are looking for a new way to grow one of Europe's favorite goodies - Sugar. Contact is made with many kingdoms along the western coast and early trade contacts are made, but ironically, the Portuguese have little to offer many of the kingdoms they reach. This due to the formation of kingdoms and empires which were already extracting gold and making their own iron. (Mali and Songhai are two examples of West African empires just inland at the time of European arrival.

Back to sugar... The Portuguese set up shop at Sao Tome and Principe Islands just west if modern day Gabon and Equatorial Guinea. They initially discover these two islands to be sparsely or completely uninhabited. This is in the early 1470s, but the islands would not be settled until the end of the 15th century. By this time Columbus had sailed the ocean blue and found that using Native Americans as labor would not work as they continue to die for various reasons.

But what the Portuguese soon found out was that the people of the African continent were resilient to many of the European diseases as well as in general a strong people and were thus capable of working on what would become the plantation system.

Which brings me to my last point. Portuguese had yet to find a method in which they could produce cash crops such as sugar at the rate which they wanted. This was do not only to the intensity of the labor and the unavailability of the labor force to do the work, but also because of the terrain in Western Europe and the climate etc. could not suffice to produce the quantity that was needed. By the end of the 15th century however, a small sugar plantation was built and and introduced African slave labor to the system. (These would have been a small number due to the small size of the plantation- look at this Sao Tome and Principe Island situation as a testing ground) Within a few decades it became clear that the system would work and was then exported to the New World.

And that's all I've got for now.

There are tons of references for this material, but this is one of mine: "Africa in world history from Prehistory to the present" by Eric Gilbert and Jonathan T Reynolds

Edit: One additional note – it is commonly assumed that once Europeans made it to the African coast they were able to immediately overpower the local kingdoms and people with in a few years. The way I left my answer it would seem that once São Tomé and Principe were settled that was the end game. This is not the case… it was not until the mid-late 16th century, about 50-100 years, before the Portuguese began to really make headway with the kingdoms and people along the coast. It was then another half a century (roughly speaking) before the African slave trade began in earnest as the one that we know of today.

Jim Walvin's "Black Ivory" is pretty good on the origins of the trans-Atlantic slave trade. But, your statement is essentially true; a slave trade existed throughout Africa and the Middle East which the Europeans then became involved in.

Hi there, there's always room for more discussion here, but you may be interested in these previous threads:

https://www.reddit.com/r/AskHistorians/comments/1rp6v4/how_morally_culpable_are_the_african/ (1 year ago, 11 comments)

https://www.reddit.com/r/AskHistorians/comments/v03ev/how_profitable_was_the_atlantic_african_slave/ (3 years ago, 35 comments)

https://www.reddit.com/r/AskHistorians/comments/2a52gp/how_important_was_the_slave_trade_to_north/ (1 year ago, 4 comments)

https://www.reddit.com/r/AskHistorians/comments/2j3gec/ohio_state_university_professor_robert_davis/ (11 months ago, 17 comments)

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