A new study by researchers at the consumer genetics company 23andMe has paired tens of thousands of genetic profiles with detailed historical records to trace the ancestry of modern-day African Americans back through the transatlantic slave trade.
The results, published Thursday (July 23) in The American Journal of Human Genetics, confirm some of what is known about where people in Africa were taken from and where they disembarked in the Americas, but they also yield some new insights. Finding some regions overrepresented among African Americans’ ancestry sheds light on secondary slave trading, while the underrepresentation of other African groups points to regional differences in the treatment of slaves. The findings also identify a sex bias in which women contributed significantly more to the gene pool of modern-day African Americans than enslaved men did.
“For millions of people in the Americas, the story of the transatlantic slave trade is basically a story of their ancestral origins,” says Steven Micheletti, a population geneticist with 23andMe and the lead author on the paper. “We really wanted to help them better understand where that ancestry came from.”
During the transatlantic slave trade—the largest forced migration in human history that took place between 1515 and 1865—an estimated 12.5 million people were forcibly removed from African countries and transported across the Atlantic to the Americas. While around 3 percent to 5 percent ultimately ended up in North America, many more disembarked in South and Central America or the Caribbean.
Reconstructing the mass movement of so many people took more than a decade to complete and involved more than 50,000 participants, making it the largest DNA study to examine African ancestry in the Americas. Genetic samples were pulled mostly from consenting 23andMe customers living along the West Coast of Africa and along the eastern coastlines of North, Central, and South America where slaves would have disembarked, with additional samples curated from public sources or collected by existing researchers in African countries lacking adequate genetic representation.
“The importance of this paper is in the great number of individual genetic profiles analyzed all at once,” says Chiara Barbieri, a human molecular anthropologist at the Max Planck Institute for the Science of Human History who was not involved in the study. Having access to such a large and robust dataset, Barbieri says, “gives you an analytical power that lends substantial weight to the paper’s findings.”
Micheletti looked for identical sections of DNA on the chromosomes of people thought to be linked through common ancestry to calculate their identity by descent (IBD). “After generations of reproduction, IBD segments get broken up into smaller and smaller pieces,” Micheletti says, adding that the rate of these breakages can be estimated using mathematical models to calculate the time since two people shared a most recent common ancestor.
The results confirm that most Americans of African descent have genetic roots in Angola and Democratic Republic of Congo, consistent with records kept of slaves disembarking in the Americas. The most recent ancestry calculations from the slave trade also showed that individuals came from West Central Africa, while the oldest ancestry was linked to the Windward Coast and Senegambia—a blanket term for what is today The Gambia, Guinea, and Senegal—mirroring the slave trade’s targeting of different regions over time.
The proportion of Nigerian ancestry in the United States was much higher than the authors expected, as historical records showed little evidence of direct transport between Nigeria and the US, and historians consulting on the project initially flagged the finding as an error. “As soon as they said that, we had to dig deeper and find out what was going on,” says Joanna Mountain, a geneticist with 23andMe and a coauthor on the paper.
The team found that towards the end of the slave trade, most slaves were being transported from Nigeria to the British occupied Caribbean before being sent throughout the Americas in a secondary, intra-American slave trade that continued long after the transatlantic trade become illegal. This intra-American trade enriched the genetic contributions of Nigerians in modern African Americans in the United States. “This is where the historical consultants came in and helped put our results into a historical context that we might not have discovered otherwise,” Micheletti says.
Fatimah Jackson, a biological anthropologist at Howard University who was not involved in the study, says that the disproportionate contributions of Nigerian ancestry in the United States could also have been the result of “breeding camps” established in states such as Virginia to maintain the population of slaves as the transatlantic trade decreased. In an email to The Scientist, Jackson says such camps “are really key to changes in the frequencies of certain African regional phenotypes once the internal slave trade was underway.”
In contrast, Senegambians were found to have a much lower genetic contribution in modern African Americans living in the US despite being an early and prolonged origin of slaves for the country. A comparison with historical records suggests that Senegambians were often sent to rice plantations because they cultivated rice in Africa. With so much standing water, these plantations experienced frequent outbreaks of malaria, and many Senegambians may have died as a result.
Jackson argues that a more thorough understanding of the history could offer other explanations. Senegambians, she says, likely weren’t “any more exposed to malaria than anyone else.” Rather, she attributes their reduced genetic presence to two other possibilities: Senegambians were known to be instigators of many notable rebellions, especially during the Middle Passage. The consequence of a failed rebellion, she notes, is death. Alternatively, many Senegambians were Muslim, meaning they were less likely to marry or have children with others outside their religion.
The study also identified a bias across the Americas in which the maternal genetic contribution was vastly overrepresented. According to the authors, roughly 15 African women had children for each African man in Central and South America and the Latin Caribbean, even though more than 60 percent of slaves were male. In the United States, this ratio was only 2:1.
This bias underscores a history of sexual exploitation of African women by men in the Americas. Mountain and Micheletti attribute the regional differences to social practices. While rape of African women did occur in the US, notions of racial purity led to less mixing between races, and slaves were often encouraged or forced to have children with other slaves to increase the labor pool. Latin American countries, by contrast, followed a common practice of racial whitening. European men were actively recruited by the government of Brazil to marry dark-skinned women with the intent of reducing African ancestry. In modern-day Latin America, the study found relatively few people in the database with greater than 5 percent African ancestry, despite the region receiving roughly 70 percent of all slaves.
While the team relied on two historians well-versed in the transatlantic slave trade—Linda Heywood and John Thornton of Boston University—to help identify which parts of Africa needed to be included and to interpret the study’s findings, Jackson says this study suffers from a common issue: while “the authors are to be commended” for their contributions, geneticists often fail to include historians early in the planning of their studies, bringing them in only at the end to explain anomalies. “Geneticists, like any scientist, believe if we have high quality data, the data will speak to us,” Jackson tells The Scientist. Instead, what is needed is for “geneticists to work more intimately with scholars who have knowledge of the historical and social context of evolutionary processes.”
S.J. Micheletti et al., “Genetic consequences of the transatlantic slave trade in the Americas,” Am J Hum Genet, doi:10.1016/j.ajhg.2020.06.012, 2020.