From Wikipedia
http://en.wikipedia.org/wiki/Atlantic_slave_trade
Origins
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Slavery was practiced in Africa
before the beginning of the Atlantic slave trade.[10]
Slavery and the slave trade were an integral part of African societies and states which supplied the Arab world with slaves for centuries before the arrival of the Europeans.[11] The African slave trade provided a large number of slaves to Europeans and their African agents.[12][13]
The Atlantic slave trade is customarily divided into two eras, known as the First and Second Atlantic Systems.
The First Atlantic system was the trade of African slaves to, primarily, South American colonies of the Portuguese and Spanish empires; it accounted for only slightly more than 3% of all Atlantic slave trade. It started (on a significant scale) in about 1502[14] and lasted until 1580, when Portugal was temporarily united with Spain. While the Portuguese traded slaves themselves, the Spanish empire relied on the asiento system, awarding merchants (mostly from other countries) the license to trade slaves to their colonies. During the first Atlantic system most of these traders were Portuguese, giving them a near-monopoly during the era, although some Dutch, English, Spanish and French traders also participated in the slave trade.[15] After the union, Portugal stayed formally autonomous, but was weakened, with its colonial empire being attacked by the Dutch and English.
The Second Atlantic system was the trade of African slaves by mostly English, Brazilian, French and Dutch traders. The main destinations of this phase were the Caribbean colonies, Brazil and North America, as a number of European countries built up economically slave-dependent colonial empires in the New World. Amongst the pioneers of this system were Francis Drake and John Hawkins[citation needed].
Only slightly more than 3 percent of the slaves exported were traded between 1450 and 1600, 16% in the 17th century. More than half of them were exported in the 18th century, the remaining 28.5% in the 19th century.[16]