How the slave trade left its mark in the DNA of people in the Americas

A study of the DNA of people in the Americas with African heritage has revealed overlooked details about the transatlantic slave trade.

“This gives some clarity and some sense of individual history,” says historian Linda Heywood of Boston University in Massachusetts, who wasn’t involved in the research. DNA evidence means African Americans can pinpoint where their ancestors were abducted from and reclaim aspects of their heritage that were hidden by the slave trade, she says. “It broadens the way in which identity and personal history can be thought about.”

An estimated 12.5 million people were taken from Africa to the Americas between the 1500s and 1800s, according to historical texts like shipping documents and records of people being sold.

To fill out the picture, Steven Micheletti of consumer genetics firm 23andMe in Sunnyvale, California, and his colleagues looked at DNA from 50,281 people, including 27,422 people from across the Americas with a minimum of 5 per cent African ancestry, 20,942 Europeans and 1917 Africans. This allowed them to identify stretches of DNA that are unique to people from particular regions of Africa.

The data came from 23andMe customers, along with public genome databases. Studies like this are becoming possible because African people, who were previously under-represented in genome databases, are now being asked to take part in research, says Joanna Mountain, also of 23andMe. Nevertheless, gaps remain. “I’m hoping we get some data from Mozambique sometime soon. It was involved in the slave trade, but we didn’t have enough data to include it in this study,” she says.

In line with historical records of where slaves were taken from, the African DNA in people in the Americas was most similar to that of people living in west African countries like Senegal, the Democratic Republic of the Congo and Angola.

However, most people in the Americas with African ancestry won’t have DNA from a single region of Africa. “Our results suggest the average African American would have connections to multiple regions,” says Micheletti. That is partly because slave traders disregarded ethnic identities, mixing people from different groups, and partly because African Americans moved around within the US. For instance, during the Great Migration of the 20^th century African Americans moved from the segregated southern states of the US to northern states.

Because so many people were abducted as slaves, much of the genetic diversity in Africa was carried to the Americas, says Eduardo Tarazona-Santos at the Federal University of Minas Gerais in Belo Horizonte, Brazil. “But within the Americas, this diversity was more homogenised between populations.”

The analysis points to overlooked details of the slave trade. For instance, the team found less DNA from Senegal, Gambia and regions in other neighbouring countries than would be expected given the huge numbers of people taken from there. This may be because those slaves were often taken to rice plantations in the US, where the death rate was high due to malaria, says the team.

Meanwhile, many people in Central and South America and on many Caribbean islands today carry little African DNA – despite the fact that 70 per cent of slaves who survived the trip to the Americas were sent there.

This may reflect a form of racism once practised in Brazil, says Mountain, in which women of African descent were raped or forced to marry Europeans to promote “racial whitening”. In contrast, in the US, African Americans were often segregated from white people by law, and racial intermarriage was illegal or taboo.

The genetic data also confirms that female slaves have passed on much more of their DNA than male slaves – even though historical records show the majority of people taken from Africa were male. This is probably because female slaves were subjected to rape and sexual exploitation.