23andMe study untangles the brutal history of American slavery

"This is a difficult event to discuss, but is the story of millions of people's ancestors living in the Americas today."

Between the 16th and 19th centuries, ships of enslaved people sailed to the Americas — an abhorrent chapter that lives on in racist policies and genetic code.

In a study published Thursday in the American Journal of Human Genetics, researchers carried out the "most comprehensive genetic investigation of the transatlantic slave trade" to date. The team combined historical records and shipping manifests, modern scholarship, and genetic data of more than 50,000 people living in the Americas and across the Atlantic coastline of Africa.

The genetic data was collected by the consumer personal genetics company, 23andMe. Its analysis illuminated genetic links spanning continents and yielded a more comprehensive understanding of the African roots of American people, the researchers say.

"What emerged from this collaborative effort was the revelation that the inequality and dehumanizing acts practiced by nations and entities involved in the slave trade explains many of the genetics patterns we observe today," study co-author Steven Micheletti, a population geneticist at 23andMe, tells Inverse.

"We hope the public becomes informed about the transatlantic slave trade as a whole and recognizes the atrocities that were occurring just recently in the 19th century," he adds.

Co-author Joanna Mountain, Senior Director of Research at 23andMe, says that for so many Americans of African descent, the details of their origins in Africa are lost.

"Genetic data may enable people to get beyond the 'brick walls' in their genealogical research and recover information critical to identity formation," Mountain tells Inverse.

What part of the Covid-19 pandemic do you think causes the most confusion? We want to know. Take the Inverse reader survey

The living past— To fill in existing holes in genealogical research around enslavement, the team compiled genetic data from consenting 23andMe research participants, as well as those participating in the 1000 Genomes Project, the Human Genome Diversity Project, and other academic studies.

Overall, participants' genetic history represented the major slave-trading regions of Atlantic Africa, Europe, and the Americas. They shared their self-described ancestry and historical family ties as well as saliva samples to extract their DNA. This data was analyzed alongside centuries of historical documents, voyage records, and other studies.

The study team zeroed in on the DNA passed down from African ancestors who survived the voyage across the Atlantic. The transatlantic slave trade forcibly displaced 12.5 million people; only an estimated 10,650,000 survived the crossing. It is the only the genes of the survivors that this study was able to incorporate.

They found that genetic evidence of Atlantic African ancestry across the Americas is consistent with historical records of transatlantic shipping of enslaved Africans. But when researchers broke down the data more granularly by specific locations, they uncovered some mismatches:

  • Genetic ancestry in the Americas from certain regions of Africa runs counter to what historical records show. The team observed much less African ancestry in Latin and South America than originally presumed.
  • Nigerian ancestry was over-represented in many parts of the Americas compared to what historical records indicate. This most likely reflects the often under-discussed active trade that moved slaves within the Americas, even after the slave trade had been banned by several European nations, the authors say.
  • Senegambia — which was a loose confederation between Senegal and Gambia that dissolved in 1989 — appears to be underrepresented given the overall proportion of enslaved people who were deported from this region into the Americas. This may be because Senegambians were transported to rice plantations in the United States. These were often rampant with malaria, resulting in an extremely high death rate.
  • Latin American individuals had strikingly lower African ancestry proportions compared to those from the United States and British Caribbean: The proportion of people with greater than five percent African ancestry was five times lower in Latin America, despite Latin, Central, and South America receiving roughly 70 percent of all disembarked African slaves. This could be due to higher mortality and smaller effective population sizes in enslaved people brought to Latin America. Another factor to consider is that many Latin American countries promoted the dilution of African heritage through intermarriage between fair-skinned Europeans and women of African descent.
  • In line with other research, the team observed a bias towards African female contributions to the gene pools of the Americas due to generations of rape and exploitation: The team estimates about 15 African women reproducing for each African man in Central America and similar values in South America and the Latin Caribbean. The evidence that African females contributed to the gene pools of the Americas to a much greater extent than did African males is "surprising" given that the majority of enslaved peoples were male, the authors say.

A new chapter — While there are numerous historical records related to the slave trade itself and treatment of enslaved people in the Americas after their disembarkation, never before has genetic data been used to "bring those historical records to life," the researchers say.

Crucially, the study illuminates the events that occurred after enslaved people set foot in the Americas, Micheletti says.

"These events, unfortunately, support historical literature suggesting various forms of mistreatment of people of African descent, and how those practices have varied across nations," he explains.

This image shows the general direction of the triangular trade routes between continents during the transatlantic slave trade.Micheletti et al, American Journal of Human Genetics

This study isn't definitive — there are still far more unanswered questions about how enslaved people were mistreated, and how these practices live on in people's brains and bodies today.

It does, however, uncover genetic links that may give people a greater sense of place and ancestral identity. Investigating and acknowledging this dark, abominable history also enables people to appreciate the preventable but precious sacrifices millions made to build the Americas.

"Though our paper focuses on painful events that have led to African ancestry across the Americas, we acknowledge the valuable contributions enslaved people and their descendants made to the formation of the nation-states of the Americas," Micheletti says. "Enslaved Africans, while enduring generations of hardships, built a world economy and influenced culture not only within the Americas but across the globe."

Abstract: According to historical records of transatlantic slavery, traders forcibly deported an estimated 12.5 million people from ports along the Atlantic coastline of Africa between the 16th and 19th centuries, with global impacts reaching to the present day, more than a century and a half after slavery’s abolition. Such records have fueled a broad understanding of the forced migration from Africa to the Americas yet remain under-explored in concert with genetic data. Here, we analyzed genotype array data from 50,281 research participants, which—combined with historical shipping documents—illustrate that the current genetic landscape of the Americas is largely concordant with expectations derived from documentation of slave voyages. For instance, genetic connections between people in slave trading regions of Africa and disembarkation regions of the Americas generally mirror the proportion of individuals forcibly moved between those regions. While some discordances can be explained by additional records of deportations within the Americas, other discordances yield insights into variable survival rates and timing of arrival of enslaved people from specific regions of Africa. Furthermore, the greater contribution of African women to the gene pool compared to African men varies across the Americas, consistent with literature documenting regional differences in slavery practices. This investigation of the transatlantic slave trade, which is broad in scope in terms of both datasets and analyses, establishes genetic links between individuals in the Americas and populations across Atlantic Africa, yielding a more comprehensive understanding of the African roots of peoples of the Americas.
Related Tags