First off, it’s difficult to say when slavery started. We have evidences of slavery in very different societies through time.

From “Slavery from Roman Times to the Early Transatlantic Trade” by William D. Phillips (1985):

“Slavery has been a constant feature of human history, appearing in nearly every part of the world. We can trace it back to the earliest civilizations of Mesopotamia and Egypt, and we can discover it in more recent societies at various levels of development. Slavery proper still exists today.”

Slavery rises in importance in Ancient Greek and Ancient Roman societies, which go from what Phillips call slaveholding societies (societies in which slavery is legal and existent) to slave societies (societies in which slaves form a very big share of the population and form the backbone for the production of very important goods). Slavery still exists in the Middle Ages, but its importance is less than in the Ancient Greek and Roman times. But what makes slavery important again is:

” (…) Slavery’s resurgence was due to the creation of the New World plantation system, which initially produced sugar, and of an expanding market economy to absorb the production of the plantations. Sugar growing and refining required intensive labor, and, in the absence of the necessary numbers of Amerindian or European workers, the Spaniards and the Portuguese turned to black slaves, acquired by Portuguese traders who tapped preexisting trading networks along the West African coast and transported their human cargoes across the Atlantic.”

Slavery existed in sub Saharan African societies, but was not as harsh as the plantation chattel slavery we saw in the American colonies. There were examples of harsh slavery before American plantations, like galley rowers in Ancient times and mining slaves. Most recent research show that sub Saharan African slavery was more malleable, and although slaves could be sold to other people, were expected to receive a plot of land of their own to till and could rise in influence over time and become free, slaveowners of their own, or even kings.

Also, it must be said that in the first contacts that the Portuguese had with the sub Saharan Africans, violence was the norm and the defeated men and women they found were made captive slaves, and brought back to Portugal. Antão Gonçalves expedition of 1441 is probably the first time there was contact between Europeans and sub Saharan Africans, and it resulted in Antão’s soldiers attacking a man who was walking on the beach with his camel and enslaving him.

These attacks were a continuation of the Reconquista, the Holy War that the Christians were waging on the Muslims in the Iberian Peninsula. In 1415, the Portuguese extended their Reconquista to the North African territory, by taking the city of Ceuta. But they were also making lots of raids along the North African coast, just like the North African Muslims were making raids on the Iberian coast (these would be called Barbary pirates later on, but piracy and raiding resulting in the taking of captives to be ransomed or made slaves was a constant in the borders between Christian and Muslim rule). The Portuguese, under their king D. Henrique, the Navigator, were making these raids more and more to the South. Some people say D. Henrique did that because he was angry that the Canary Islands had been granted to the Castillians by the Pope in the 1300s (their native population was Pagan), and wanted to ensure that the Pope would grant the African lands to Portugal. Several Papal Bulls (texts published by the Papacy) would grant this monopoly of the conquest of African lands and African people to the Portuguese , and the right to enslave any Pagans or Muslims they found there (Dum Diversas, Inter Caetera, among others). Also, to further reinforce their connection to the Reconquista/Crusade/Holy War, people would be pardoned of their sins if they went in these Portuguese expeditions.

These attacks were made on boats called caravelas, that had a crew of between 20 to 30 people. Most of the time it was one boat or a few boats, but the outfitting of such enterprises was costly. The king, and the other people who financed such an expedition, expected some kind of financial return. Part of it was received by trade. Antão Gonçalves expedition was supposed to get sealion oil and furs, but also got slaves, which was another way to make the enterprise pay for itself. Between 1441 and 1450, something like 1,000 slaves were brought to Portugal in these raids. That’s the beginning of the African slavery by Europeans.

But this was still small. The slave trade starts to get bigger in the next step. The Portuguese explorers then encounter very big African realms and their raids were not that successful. So they start to trade with these African realms. At the same time the Portuguese discover islands in the Atlantic that are non inhabited: Madeira, Sao Tome, Principe and Cabo Verde. The Portuguese at first tried to settle these islands like European land, with crops like wheat and such. It failed, so D. Henrique tried commercial crops. The Portuguese had learned from the Muslims they conquered how to plant some spices, especially sugar (and other Europeans also learned that due to contact with Muslims and helped the Portuguese in this, like Genoa). The Europeans were trying to make their own production of sugar, since it came from the East and was very, very expensive at the time. The way the Muslims produced it was through lots of slave labor in very hot environment. When the Portuguese tried to plant sugar in these non inhabited islands, they needed the manpower to work in slave farms. And they got it from trading with the African kingdoms. And this is how a slave society was born, and the slave trade actually got that trade part.

Later, when the Spanish and the Portuguese discovered the Americas, there was the initial enslavement of the American natives. But for several reasons, this would not be the main way the American colonization would happen, and they would resort to the importation of African slaves, which would skyrocket the slave trade. Native American slavery was tried, and it worked in many areas, but there were important actors that didn’t want it: the Crown and the Catholic Church. A big debate happened between the priests on whether the Natives had a soul or not, and in the end it was decided they had, and that their enslavement was illegal. It could become legal only if it was due to a “just war”. “Just war” in this case was comprised of two things: if the Natives attacked the Europeans, it was considered just for the Europeans to enslave these “savages”. Also, if Native had enslaved another Native and sold to an European, that was also considered legal, since the act of enslavement wasn’t done by an European. In my part of Brazil (Sao Paulo), this resulted in two things: Europeans encouraging Native to fight Native, acquiring the slaves of such wars, and the mestizos, the people who were part Native and part European, attacking the Natives. The priests, who didn’t really liked what was happening, even called these mestizos “mamelucos”, after the mamluks, the slave soldiers that existed in Muslim lands and fought for Muslim rulers, even though their parents were Christians or Turks or some other non-Muslim group. Anyway, my part of the country was a backwater, and Native enslavement only existed in backwater areas (like Paraguay). In the parts in which there big populations and civilizations, like Mexico or Peru, the Europeans adapted the preexisting labor arrangements with the Spanish governor where the Native leaders used to be before. In other areas, which were not backwater but were also not that much populated, either because there weren’t many Natives before, or because the Natives died due to disease or ran away to the hinterland fleeing the Europeans, African slavery was introduced. And this became a very profitable business, not only for the people who employed the slaves in plantations in Brazil, the Caribbean coast or the American South, but also for people who bought slaves in Africa and transported them to the Americas. And this thing was more “tax-able” for the Crowns of the time than a bunch of bandeirantes going by themselves to attack some Native tribe in the middle of Brazil. Just set some authority in the main ports then you’ll get your taxes when the boats arrive to sell the slaves in the slave markets. Not to say it was only about taxes (the difference between how the colonials wanted to treat Natives, mostly harshly, and how the Crowns wanted to treat them, not that harshly, appeared many times in the history of the Americas, either that be British, Portuguese or Spanish), but it was a factor.