From "Chronology on the History of Slavery 1619 to 1789"

The Atlantic slave-trade was different from all these earlier slavery in several respects. Most enormously important is that it was the first form of slavery that was solely motivated by commercial incentives. In earlier times slaves were used as domestic workers and soldiers, since there were no plantations or industrial factories where millions of slave-labor was needed. The African slave-trade was a capitalist invention. Readers are directed to Slavery and Capitalism by Eric Williams.

It was the large-scale capitalist mode of production which required cheap labors that induced the slave trade. It was the Industrial Revolution in Europe that made it necessary to traffic in human lives on a colossal scale.

Slaves in earlier times enjoyed social and individual rights - like marriage, freedom to raise a family, speak their language and worship their gods, rights which were denied the African slaves exported to the Americas. Africans captured and taken into the new world were stripped of all their personality and humanity - they could not even bear their own names.

From "Chronology on the History of Slavery 1619 to 1789"

Be prepared to read... for a long time... but it is worth it.