The legacy of the transatlantic slave trade can still be found in the DNA of people living across the Americas, as shown by a new study published in the American Journal of Human Genetics on Thursday.

Scientists from the biotech giant 23andMe took a deep dive into the DNA of over 50,000 people, 30,000 of who have close African ancestry, to lay bare the history of slavery in the Americas and the deep hardship faced by millions of people. The study – described by the researchers as “the most comprehensive investigations of the transatlantic slave trade” – affirms much of what’s already known through historical records and written sources from those who endured the experience, although it does also illuminate a number of unexpected findings and disturbing new insights. 

"This paper conveys how the racist and dehumanizing acts endemic to the slave trade led to different patterns of African ancestry across the Americas that we can see in the DNA of people living today. We hope readers grasp not only the impact of the slave trade but also the deep contributions enslaved Africans made to the history, economy, and culture of the Americas," Steven Micheletti, first study author and population geneticist at 23andMe, said in a statement. 

One disconcerting insight was the difference between the levels of African ancestry found in people living in Latin America and the US. Despite Latin America receiving roughly 70 percent of all disembarked previously enslaved people, the proportion of people with greater than 5 percent African ancestry is five times lower in Latin America than in the US. The researchers argue this peculiar paradox is most likely a reflection of the different grim practices adopted by slave-owners and governments across America. 

“Many slave-owners in the United States promoted enslaved people having children with one another for the purpose of maintaining a workforce, and even after slavery, they tended to segregate people of African descent," added Micheletti.

Alternatively, parts of Latin America adopted a policy of so-called "dilution" after the abolition of slavery in an attempt to weaken African heritage in the population. Micheletti explains: "In the early 1900s, sources state that the Brazilian government implemented immigration laws seeking to bring more Europeans into the country, presumably to have children with darker-skinned females and reduce African ancestry.”

It was also found that African women are notably more prominent in the gene pool compared to African men, which is particularly surprising since most enslaved people were men. Perhaps most grim of all, this peculiarity highlights that enslaved men often died before they had a chance to have children and pass on their genes, while enslaved women were often raped and forced to have children.

Just as other sources have highlighted, the data found most Americans of African descent have some genetic roots in present-day Angola and the Democratic Republic of the Congo. However, it also indicates that much more Nigerian ancestry is present in African Americans than previously suggested.

They also found that genetic connections between African Americans and Senegambians from present-day Senegal and the Gambia were much lower than expected. Horrifically, this is not purely because fewer people from this area were forced over to the Americas. 

"Because Senegambians were commonly rice cultivators in Africa, they were often transported to rice plantations in the US. These plantations were often rampant with malaria and had high mortality rates, which may have led to the reduced genetic representation of Senegambia in African Americans today," Micheletti explained.