"More than one and a half centuries after the trans-Atlantic slave trade ended, a new study shows how the brutal treatment of enslaved people has shaped the DNA of their descendants.
The report, which included more than 50,000 people, 30,000 of them with African ancestry, agrees with the historical record about where people were taken from in Africa, and where they were enslaved in the Americas. But it also found some surprises.
For example, the DNA of participants from the United States showed a significant amount of Nigerian ancestry — far more than expected based on the historical records of ships carrying enslaved people directly to the United States from Nigeria.
At first, historians working with the researchers “couldn’t believe the amount of Nigerian ancestry in the U.S.,” said Steven Micheletti, a population geneticist at 23andMe who led the study.
After consulting another historian, the researchers learned that enslaved people were sent from Nigeria to the British Caribbean, and then were further traded into the United States, which could explain the genetic findings, he said.
The study illuminates one of the darkest chapters of world history, in which 12.5 million people were forcibly taken from their homelands in tens of thousands of European ships. It also shows that the historical and genetic records together tell a more layered and intimate story than either could alone.
The study, which was published on Thursday in the American Journal of Human Genetics, represents “real progress in how we think that genetics contributes to telling a story about the past,” said Alondra Nelson, a professor of social science at the Institute for Advanced Study in Princeton, N.J., who was not involved in the study.
Although the work is commendable for making use of both historical and genetic data, Dr. Nelson said, it was also “a missed opportunity to take the full step and really collaborate with historians.” The history of the different ethnic groups in Africa, for example, and how they related to modern and historical geographic boundaries, could have been explored in greater depth, she said.
The study began as a dream project of Joanna Mountain, senior director of research at 23andMe, even before the company had any customers. Over 10 years she and her team built a genetic database. Primarily the participants were 23andMe customers whose grandparents were born in one of the geographic regions of trans-Atlantic slavery. All participants consented to have their DNA used in the research.
In the new study, Dr. Micheletti’s team compared this genetic database with a historical one, Slave Voyages, which contains an enormous amount of information about slavery, such as ports of embarkation and disembarkation, and numbers of enslaved men, women and children.
The researchers also consulted with some historians to identify gaps in their data, Dr. Mountain said. Historians told them, for example, that they needed representation from critical regions, like Angola and the Democratic Republic of Congo. The team worked with academics connected to West African institutions to find that data. The size of the project’s dataset is “extraordinary,” said David Reich, a professor of genetics at Harvard who was not part of the project.
Because it drew participants from a direct-to-consumer database of millions of people, the study was able to “ask and answer questions about the past and about how people are related to each other” that could not be asked by academics like himself, he said. At best, academic projects are able to study hundreds or a few thousand people, and generally that data does not also include the genealogical information that the 23andMe research participants provided.
The findings show remarkable alignment with the historical record. Historians have estimated, for example, that 5.7 million people were taken from West Central Africa to the Americas. And the genetic record shows a very strong connection between people in West Central Africa and all people with African ancestry in the Americas.
Historians have also noted that the people who were taken to Latin America from Africa disembarked from West Central Africa, but many were taken originally from other regions like Senegambia and the Bight of Benin. And the new genetic evidence supports this, showing that the descendants of enslaved people in Latin America generally carry genetic connections with two or three of these regions in Africa.
Historical evidence shows that enslaved people in the United States and the British Caribbean, by contrast, were taken from a larger number of regions of Africa. Their descendants today show a genetic connection to people in six regions in Africa, the study found.
The historical record shows that of the 10.7 million enslaved people who disembarked in the Americas (after nearly 2 million others died on the journey), more than 60 percent were men. But the genetic record shows that it was mostly enslaved women who contributed to the present-day gene pool.
The asymmetry in the experience of enslaved men and women — and indeed, many groups of men and women in centuries past — is well understood. Enslaved men often died before they had a chance to have children. Enslaved women were often raped and forced to have children.
The 23andMe project found this general pattern, but also uncovered a startling difference in the experience of men and women between regions in the Americas.
The scientists calculated that enslaved women in the United States contributed 1.5 times more to the modern-day gene pool of people of African descent than enslaved men. In the Latin Caribbean, they contributed 13 times more. In Northern South America, they contributed 17 times more.
What’s more, in the United States, European men contributed three times more to the modern-day gene pool of people of African descent than European women did. In the British Caribbean, they contributed 25 times more.
This genetic evidence, the scientists say, may be explained by local practices. In the United States, segregation between enslaved people and the European population may have made it more likely that the child of an enslaved mother would have an enslaved father. But in other regions where enslaved men were less likely to reproduce, dangerous practices like rice farming — in which harsh conditions and muddy fields made it easier to drown, and malaria was common — may have killed many of them before they could have children.
In some regions in Latin America, the government enacted programs that brought men from Europe to father children with enslaved women in order to intentionally diminish the African gene pool.
The study illustrates how much physical and sexual violence were part of slavery — and how they are still built into our society, Dr. Nelson said. It confirms the “mistreatment, discrimination, sexual abuse, and violence that has persisted for generations,” she said, and that many people are protesting today.
Correction: July 23, 2020. An earlier version of this article was imprecise in discussing some historical research. Historical records do exist showing that enslaved people were shipped directly from Nigeria to the United States, but not to the extent indicated by the new genetic evidence.