Until less than two centuries ago, millions of people were forcibly removed from Africa by European colonisers and taken across the Atlantic to become slaves in the Americas.

Helping to unravel the pervasive, ongoing impacts of this enslavement, an extensive population genetic study has added to historical records gathered by the slave trade database, concurring with them overall but adding some new insights.

For millions of people in the Americas, the story of the transatlantic slave trade is the story of their ancestral origins, says Steven Micheletti from 23andMe in Sunnyvale, US, and lead author of the study published in The American Journal of Human Genetics.

The team, which included a researcher from the University of Leicester, UK, analysed genetic data from more than 50,000 people on both sides of the Atlantic, working closely with historians, scholars of African American studies and other geneticists.

As predicted, they found strong genetic connections between people of the Americas and African regions where more people were enslaved, most with roots in Angola and the Democratic Republic of the Congo.

But a closer look revealed multiple deviations from the groups expectations, says Micheletti.

One discovery was that most US-based African Americans tend to have high Nigerian ancestry, even though relatively small numbers of their enslaved ancestors were taken directly to the US from present-day Nigeria.

This supports historical accounts of many enslaved people being transported across the Americas after the transatlantic slave trade was abolished, explaining why they found many distant relatives between the Caribbean and the US.

In contrast, African Americans had lower genetic connections with Senegambians than would be expected, possibly because these people tended to be transported to work in rice plantations that had high death rates from malaria.

Another important and tragic finding was the genetic sex bias in African women, showing they reproduced more than African men even though more than 60% of enslaved individuals were male.

For every African man in Central and South America and the Caribbean, the analysis found that about 15 African women had children.

This reflects known accounts of rape and exploitation of female African slaves, says Micheletti. Unexpectedly, the sex-bias was nearly 10 times larger in Latin America than the US, he adds, which could be explained by records of differences in systemic racism.

In the US, enslaved people were segregated and allowed to have children, a likely means of maintaining an enslaved workforce, known as slave breeding.

Conversely, other countries in Latin America, such as Brazil and Cuba, promoted racial whitening through immigration programs to encourage mating of white European males with dark-skinned African females to produce lighter skinned children and dilute African heritage.

This, and higher mortality, could explain why the proportion of people with more than 5% African ancestry was five times lower in Latin America, says Micheletti, even though more than two thirds of enslaved Africans disembarked there.

His goal is for the study to help African Americans find their roots and promote understanding of how their ancestors helped shape their communities.

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.

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Genetic consequences of the slave trade - Cosmos