In the mid-15th century, the ships of the trans-Atlantic trade system went from carrying cargoes of gold to carrying cargoes of human beings. Over the next 350 years, some 12.5 million people would be shipped as slaves from Africa.

Historian David Eltis has summarized that piece of history in Atlas of the Transatlantic Slave Trade, an online database that uses thousands of documents to offer a clearer picture of the trans-Atlantic slave trade. He and fellow historian David Richardson have also published a book on the subject by the same name.

Eltis and Richardson tell NPR's Neal Conan that one of their most striking findings was that just 4 percent of trans-Atlantic shipping traffic carried slaves to what would become the U.S.

"When the slave trade came to an end, the number of people living in North America that were enslaved was larger than anywhere else," Eltis says. "And it's extraordinary that the slave trade accounts for such a small share of that number."

Eltis goes on to explain that because North America had relatively benign living conditions, black populations did reasonably well -- though not as well as the white populations. Still, Eltis says North American slaves fared better than their counterparts in the Caribbean and Brazil.

Because of this, Eltis says, the growth in the slave population in North America was more the result of natural population growth than an active slave trading industry.

Another common misconception the historians debunked is that the slave trade in North America was mostly about cotton; in fact, sugar was the main driver.

In the antebellum American South, cotton was a major agricultural product while, according to Richardson, much of Brazil and the West Indies concentrated on sugar production. But conditions were harsh in the sugar sectors, and while Brazil and the West Indies boasted large numbers of slaves, those slaves had a harder time reproducing.

"I think that's part, if you like, of the whole tragedy of the slave system," Richardson says, "its wastefulness of human life."

Eltis and Richardson say the last slave ship to the U.S. landed in Mobile, Ala., in 1860. The last one to cross the Atlantic set sail in 1867 on its way to Cuba, where slavery would continue until 1888.

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