Evidence of some of the horrors of the trans-Atlantic slave trade can still be found in the DNA of African Americans today, according to new research.
Researchers from consumer genetics company 23andMe studied genetic data of more than 50,000 people and compared it to historical documentation of where people were taken from in Africa, and where they were enslaved in the Americas, said Steven Micheletti, a population geneticist at 23andMe and study co-author.
The genetic results largely matched the historical records, Micheletti said, noting that historians estimated 5.7 million people were taken from West Central Africa and his team found the strongest genetic connection between people in that region.
But researchers also found key differences that may shed light on the brutality of slavery and how it operated different in different regions, according to the study published on Thursday in the American Journal of Human Genetics.
Micheletti, for example, said he was surprised to learn that African Americans have a higher proportion of African ancestry than people of African descent in South America, even though many more enslaved people were sent to South America than the United States.
He said the potential explanation could be "two different horrible practices": In places like Brazil and Cuba, slave owners were more likely to let slaves die than worry about their health, while in the U.S., they would "essentially breed people" to maintain the enslaved workforce.
Genetic data also shows that enslaved women contributed to the present-day gene pools at a higher rate, despite the fact that more than 60% of the enslaved people who were brought to the Americas were male. In the U.S., African women contributed to the gene pool about 1.5 times more than African men. In Central America, the Latin Caribbean and parts of South America enslaved women contributed to the gene pool about 13 to 17 times more.
The biases in the gene pool toward enslaved African women and European men can be attributed to the well-documented generations of rape and sexual exploitation against enslaved women by slave owners, study authors wrote, but the significant differences between the U.S. and Latin America was a surprise.
Micheletti explained that higher mortality rates among enslaved men and racial whitening policies in Latin America are potential explanations for the discrepency. In the U.S., slave owners promoted segregation in addition to coercing enslaved people to have children.
Far more people in the U.S. and Latin American had Nigerian ancestry than expected based on historical records. This discrepancy is the result of the intercolonial slave trade that occurred between the British Caribbean and other parts of the United States between 1619 and 1807 "presumably to maintain the slave economy as transatlantic slave trading was increasingly prohibited," the authors wrote.
"These voyages would spread African ancestry common in the British Caribbean to other regions of the Americas that were not in direct trade with specific regions of Africa," according to the study.
While Nigerian ancestry was over represented, ancestry from the region of Senegal and Gambia, one of the first African regions from which people were enslaved, is underrepresented, according to the study. Study authors suggest that over time, more and more children from the region were enslaved and that pattern combined with unsanitary conditions led to lower rates of survival.
It's also possible that Senegambians died more often because they were taken to dangerous rice plantations which were generally rampant with malaria.
Micheletti said he hopes to explore these hypotheses for the discrepancies more deeply.
"We offer a lot of these potential explanations but we need to go and physically test those explanations," he said. "We’d also like to kind of shape results and put them into a more personal context for 23andMe customers."