The first Portuguese caravels edged down the Atlantic coast of Africa in the 1430s and 1440s, reaching the Senegal River by 1445. In so doing, they opened another route that paralleled the trans-Saharan roads. The Portuguese were more interested in gold than slaves, although they were willing to trade in pepper, ivory, and other products. Their investment in this exploration of the African coast was a conscious effort to bypass Muslim middlemen; initially, the Portuguese only succeeded in expanding the existing trans-Saharan trade by opening the maritime routes; later, they participated in the internal African trade as middlemen themselves. Structurally, therefore, both the Portuguese and the Muslim traders filled the same functions: they connected sub-Saharan Africa with the Mediterranean world and they participated in the regional commerce of West Africa.

The early slave trade of the Portuguese demonstrates this similarity to the established commercial patterns in four ways. Firstly, some slaves were taken to southern Europe for employment as domestic servants, a demand like that in the Islamic countries of North Africa and the Middle East. Secondly, other slavers were sold to sugar planters on islands in the Mediterranean, and when sugar production spread into the Atlantic, to buyers on Madeira, the Canaries, and the Cape Verde islands. In both cases--domestic servitude and sugar production--the market was already satisfied to some extent by the trans-Saharan trade. Thirdly, the Portuguese bought and sold slaves along the West African coast, merely transferring slaves from one place to another so that gold could be purchased as a result of the profits realized from the transit trade. Again, Muslim merchants in the interior of West Africa did the same thing. Finally, the link to the trans-Saharan trade is particularly evident in the commodities exported to West Africa by the Portuguese; these included textiles and other goods bought in North Africa. Cadamosto, who participated in a Portuguese expedition to the Senegambia in 1455-1456, was well aware of this competition with the trans-Saharan trade, evident in his observation that the Wolof king ‘sells many [slaves] to the Azanaghi merchants in return for horses and other goods, and also to the Christians, since they have begun to trade with these blacks.’

The total volume of the Portuguese slave trade along the Mauritanian, Senegambian and upper Guinea coast was about 80,000 for the second half of the fifteenth century. The 800 or so slaves bought and sold each year largely fitted into other, established commercial patterns that the Portuguese were able to tap. In the Senegambian region, for example, the Portuguese were able to buy gold, which came from Bambuhu and Buré in the interior, which were important sources of gold before new and larger deposits were discovered in the Americas. Another reason was that trade was well established in the Senegambia, and the particular geographical features of the region made it easy for sea-borne merchants from Portugal to tap existing commerce. Both the Senegal and Gambia Rivers were navigable inland, and they crossed the routes north into the Sahara and to the Mediterranean. It was possible, therefore, to divert some trade to the coast, including gold, slaves, and other goods. Major salt sources were located near the coast north of Senegal and the Gambia. Consequently, Muslim merchants were already moving goods--salt, slaves, textiles, fish, and grain--back and forth between the coast and the interior. It was easy to introduce European commodities in order to buy gold and slaves. This trade continued throughout the whole period of the Atlantic trade, sometimes amounting to a few thousand slaves per year.

Elsewhere in West Africa, the Portuguese found an active trade in slaves on the Gold Coast, where they were also able to purchase gold. Muslim merchants had established commercial links this far south in pursuit of the same gold and, as with the Senegambia, they were on the coast itself. Gold was found in alluvial washings throughout the forest, so that it was easy once again for the Portuguese to buy gold there. The Muslim network was long established, however, so that the Portuguese dealt with these merchants as well as the actual producers of gold. The Muslim traders took gold north from the Akan deposits towards Timbuktu and thence across the Sahara. They traded in many other goods, too. They brought salt, livestock, textiles, and copper-ware south, and to the north they took kola nuts which were widely consumed in the savanna as a stimulant. The Portuguese imported slaves to buy gold. Perhaps, ironically, the first European slave-traders on this stretch of coast became involved as carriers in the domestic slave trade of West Africa. As was the case in the Senegambia, therefore, the early contribution of the Portuguese to the slave trade was to modify existing patterns of commerce. The Portuguese began purchasing slaves in the ‘slave rivers’ of the Benin coast sometime in the 1470s, and the 1480 expedition acquired 400 slaves. By the early sixteenth century, the trade between the Bight of Benin and the Gold Coast was in the order of 500 to 600 slaves per year. These were channeled through the Portuguese depot at São Jorge da Mina, later to be called Elmina. These slaves were sold to African merchants for use as porters in the trade with the interior, although a few were kept at São Jorge da Mina as labourers.

In the first two decades of the sixteenth century, slave exports totaled about 2,300 to 2,500 per year, rising to 4,500 per year in 1515-1521, a level that was maintained at least until the mid-century. The increase was related to the inclusion of the Kingdom of Kongo into the commercial network of the Portuguese and to the development of the island of São Thomé, located off the shore in the Gulf of Guinea, as a transit point for the slave trade and a plantation centre for the production of sugar cane. These developments were a further extension of existing commercial practices. São Thomé became another in a string of depots; slaves were brought form Kongo to the island for trans-shipment to the Gold Coast, Madeira, the Cape Verde islands, and Portugal. Between 1510 and 1540, four to six slaving vessels were kept continually occupied hauling slaves to the Gold Coast, for example. Cargoes varied in size from caravels that could carry 30 to 80 slaves, to larger vessels that could transport as many as 100 to 120 slaves.

The emergence of a trade that was independent of the Muslim sector was closely associated with the Kingdom of Kongo, the major state on the coast of central Africa when the Portuguese arrived there in 1482. King Afonso I (1506-1545) formed an alliance with the Portuguese crown, through their representatives at the Kongolese capital of São Salvador and on the islands of São Thomeé and Principé. Afonso attempted to introduce reforms designed to increase his power, including the acceptance of Christianity and the reliance on Portuguese advisors. Afonso used the slave trade to promote the interest of the state. Through border skirmishes and tribute collection, the state acquired slaves for export. Afonso even managed the transport of some slaves to São Thomé through the agency of two members of the royal household who were stationed in São Thomé in 1526.

These efforts at centralization were only partially successful; indeed the recurrent theme of Kongolese history in the sixteenth century was one of conflict between royal attempts at monopoly and the activities of private traders, both Portuguese and Kongolese. On the one side, Afonso directed trade through São Salvador and the port of Mpinda, near the mouth of the Zaire River. Except for the provincial governors of Ndongo, in the south, the nobility cooperated with these royal efforts. A board of enquiry protected official interest by investigating alleged kidnappings, which attempted to restrain the movement of private traders. Private traders, on the other hand, were not to be outdone. The clients of Fernão de Melo, who had the Portuguese charter for São Thomé, and Antonio Carneiro, who had the charter for Principé, bought slaves wherever they could. By the 1520s, their agents traveled into the interior beyond São Salvador, probably as far as Malebo Pool (Stanley Pool) on the Zaire River and Ndongo in the south.

The principle conflict was between slave traders, official and private, and not between Kongo and Portugal, despite occasional differences over the autonomy of the Catholic Church in Kongo and other issues. This struggle over the control of trade facilitated the growth of slave exports. In the 1520s, exports from Mpinda were in the order of 2,000 to 3,000 per year. By the 1530s, the volume had increased from 4,000 to 5,000 in some years, and in 1548, to from 6,000 to 7,000. most of these slaves were sent to São Thomé, from where they were redistributed elsewhere or used on local plantations. São Thomé began exporting to the Americas in the 1530s, and by 1550 the bulk of the transit trade was destined for the Spanish Caribbean. Thereafter, the trade to the Americas predominated until the decline of São Thomé at the end of the sixteenth century.

The Kongo-São Thomé axis collapsed in the last third of the sixteenth century. In 1568, invaders from the interior penetrated the kingdom, even occupying São Salvador until a combined Portuguese-Kongolese expedition relieved the capital. This weakened Kongo temporarily, which worked to the advantage of the private traders. In 1576, the Portuguese crown, interested in containing the interlopers itself but relying less on the Kongo monarchy than previously, established a base at Luanda Bay, which had become the main avenue into the interior south of Kongo. Now slaves were shipped directly to the Americas from Luanda, as well as from São Thomé. Official Portuguese efforts appear to have impeded the interlopers somewhat; before the establishment of Luanda, this southern trade from Ndongo was estimated at 10,000 slaves in peak years; after 1576, exports declined, although the government share increased. The private traders were forced to bypass the Portuguese stations, just as they had previously bypassed Kongolese capital at the height of Kongolese control. As a result, the trade as a whole probably remained at about the same level, that is, several thousand slaves per year.

From 1450 to 1600, about 410,000 slavers were exported from Africa via the Atlantic basin, including those ending up on São Thomé and other islands off the African coast. Those slaves destined for internal African markets gradually became a smaller proportion of the total volume. The trade was about a third of the volume of the external trade across the Sahara, Red Sea, and Indian Ocean in the same period. None the less, the development of the trans-Atlantic trade and the tremendous expansion beginning in the last half of the sixteenth century resulting from the shipment of slaves to the Americas represent a major departure in the history of the slave trade.