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Genetic Study Reveals New Insights On Transatlantic Trade Of Enslaved People

A display about enslavement  in Mobile, Alabama. (Jay Reeves/AP)
A display about enslavement in Mobile, Alabama. (Jay Reeves/AP)

Editor’s note: This segment includes mentions of sexual violence.

Until recently, much of the information available about where enslaved people were captured before being brought to the Americas came from shipping logs and databases.

These sources detailed ports of embarkation and numbers of people transported, and new data drawn from genetics corroborates much of what historians already knew.

But the data also reveals some revelations — both about the intercolonial trade of enslaved people and some of slavery’s most brutal atrocities.

The study, which appears in the American Journal of Human Genetics, drew on genetic data from 50,000 23andMe users who agreed to lend their DNA.

The project was started by Joanna Mountain, a geneticist at 23andMe and former Peace Corps volunteer in Kenya who developed a deep interest in what happened to people who were wrenched from that continent into slavery. Mountain joined forces with population geneticist Steven Micheletti on the study that has been years in the making.

The pair worked closely with historians to compare a database of transatlantic slavery records to the genetic connections between people in Africa and people in the U.S. with African ancestry.

They found that there was a “very strong bias toward African women contributing to the present-day gene pool compared to African men,” and that the bias varies across different regions of the Americas, Mountain says.

In parts of Central America, enslaved African women were contributing 13 to 17 times more to the current day gene pool than enslaved African men, she says.

“In discussing this extreme finding with historians and looking through the historical literature, we found indications that men were far less likely to reproduce, partly because they were at higher risk of dying early if they were having to participate in rice farming or sugar cane farming, which was very risky,” she explains.

At the same time, enslaved African women were often being forced to reproduce. In parts of Latin America, “there would be initiatives to encourage European men to father children with African women in order to do something that they called dilute the African gene pool,” she says.

The rape of enslaved African women — who were sometimes forced to have children before arriving in the U.S. — as a strategy to manipulate the gene pool made their genes “more extreme” in certain areas.

“This is something that we often think of as the past. And yet the repercussions certainly continue to today in the DNA,” she says. “It’s kind of amazing that we see it so strongly in the genetic evidence.”

Interview Highlights

On the over-represented number of Americans with Nigerian ancestry, which the study found is “likely due to the intra-continental slave trade”

Stephen Micheletti: “We see a lot of U.S.-based African Americans with ancestry from Nigeria and specifically connections to the ethnolinguistic groups, the Yoruba, Esan and Igbo. And why this was so surprising is the transatlantic shipping records indicate that people around Nigeria weren’t directly arriving in the U.S. in high numbers. But what we have to realize is that enslaved people were being forced between and within the Americas over centuries. So not all the movement was happening across the Atlantic. It was also happening between all these countries and within all of these countries.”

On how few Gambian and Senegalese descendants of enslaved people there are in the U.S.

Micheletti: “We merge these two regions and call them Senegambia. And we didn’t see a lot of Senegambian representation in the U.S. and again, that’s another deviance from our expectation given the shipping records. The working hypothesis is Senegambians were typically Asian and African rice cultivators back in Africa and the Europeans were well aware of this. So it’s likely that Senegambians that arrived in the United States ended up on rice plantations because of their expertise. And what we know about rice plantations is that they were some of the most dangerous working conditions for enslaved people. One reason is malaria is a huge problem in these swampy rice fields, and there’s also other risks like drowning”

On the response from 23andMe volunteers

Joanna Mountain: “We definitely got at quite a bit of response. And it’s provoked some debate regarding enslavement and forced migrations. But also people seeing this story in their own lives. One individual said that ‘whenever I’m in Jamaica and New Orleans, I feel a sense of connection.’ And for that individual, this study felt very real to him. And so that was super exciting for us.”

On people discovering the study and making connections between European DNA and their ancestry

Micheletti: “One goal of this study was first to produce the data and then look at the historical records that best support those data. And fortunately, the records that support those data are these horrible atrocities. And with that, we wanted to make readers aware of these atrocities because they’ve shaped the genetic landscape across the Americas. And we want people to be aware of the number of enslaved people that were impacted.

“… I would say [people’s] eyes are opened by this study because a lot of people may have not heard about this history in the past. And I’ve received some personal messages that people are happy that we’re looking into the history and not just providing a genetic study, but more of a collaborative study with historians and not ignoring all of these atrocities because it’s part of their past. People have discovered using services like ours that they have European ancestry and now they have better context for why that is.”

Karyn Miller-Medzon produced and edited this interview for broadcast with Tinku RaySerena McMahon adapted it for the web. 

This article was originally published on

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