The genes of 50,000 descendants of slaves reveal the effects of the global slave trade generations later, according to a study published Thursday in the American Journal of Human Genetics. Researchers analyzed data provided by thousands of 23andMe customers who agreed to share their genetic information to better understand the impact of forced migration on the genealogy of the descendants of enslaved Africans in the Americas.
They found that enslaved people who were brought from one African region to a particular region in the Americas generally ended up sharing a genetic connection to that African region generations later, said Steven Micheletti, a population geneticist with 23andMe and the first author of the study.
But, in some cases the results did not line up with the historical records. For instance, while African Americans, based on migration documents, should show genetic roots closely linked to current day Angola and the Democratic Republic of Congo, many actually show closer genetic links to Nigeria.
High percentages of Nigerian ancestry in African Americans in the U.S. may be linked to the number of slaves who were transferred from the British Caribbean to the United States. This was backed up by historians who cited a “database on the intra-American slave trade, which made it clear that slaves had been brought from the Caribbean to the U.S.,” said Joanna Mountain, 23andMe’s senior director of research. “When you look back at the pattern of slaves brought to the Caribbean, especially the British Caribbean, you see that that was often from Nigeria.”
Dr. Bernard Powers, historian and director of the Center for the Study of Slavery in Charleston, part of the College of Charleston in South Carolina, suggests that the origin of slaves shipped from the Caribbean to the U.S might be difficult to track. But he said the genetic discrepancy may be the result of migrations by people from what is now southeastern Nigeria to parts of Angola and Congo who were later captured and sent to America.
There could have been “internal developments within the African continent, which shaped the exportation” of slaves, Powers said. “Each one of these regions has its own political and economic, as well as climate history, and the variations could contribute to the exportation of people on the coast.”
Powers and the researchers agree that once slaves from Senegal and Gambia, an area also known as Senegambia, arrived in the U.S, they suffered high mortality rates on rice plantations where malaria and gruesome work conditions were common. “Rice was really the most labor intensive crop produced in Colonial America, for sure,” Powers said. “It would have been the mainland’s approximation to sugar cultivation in the Caribbean.”
Mountain suggested that the high mortality rates may have contributed to reduced genetic representation of enslaved people from Senegambia, Angola, and the Democratic Republic of Congo in African Americans.
Despite differences in the practice of slavery among countries and colonies across the Americas, researchers also found an overarching sex-bias that appeared throughout the continents.
“Sex-biases is basically the ratio of African women that were reproducing to African men,” said Micheletti. “African women were reproducing way more than African men were. That's indicative of rape and exploitation that has been documented in diaries and other historical literature.”
The researchers recognize that their data lacks representation of global populations due to 23andMe’s mostly U.S. customer base. Yet, they say looking at genetics through the lens of historical data could bring awareness to new truths about ancestry.
“We don't want these historical details to get swept under the rug,” Micheletti said. “We really want them to be discussed today, and adding the genetic confirmation on to those details could be a powerful tool.”CORRECTION (July 23, 2020, 6 p.m. ET): A previous version of this article misstated in one reference where in Africa researchers say slaves to the U.S. came from. It was Senegal and Gambia, an area they've labeled Senegambia, not Angola and the Republic of Congo. (The researchers made the correction after publication.