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.