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Genetic Study Validates, Expands History of Transatlantic Slave Trade

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NEW YORK – A new genetic study is providing novel insights into the transatlantic slave trade in the Americas and a better understanding of the African ancestry of several American populations.

An estimated 12.5 million people from Africa, including children, women, and men, were forcibly enslaved and deported to the New World between the 16th and 19th century. Previous genetic studies have shown that most African Americans in the United States have more ancestry from populations near present-day Nigeria than from other populations in Atlantic Africa.

However, according to the new study, published in the American Journal of Human Genetics on Thursday, many of these preceding studies provided an incomplete understanding of population interactions during and after the slave trade era because they lacked representation of West Central Africa, which was the largest source of enslaved people during the transatlantic slave trade.

The new work represents one of the most comprehensive studies of the transatlantic slave trade and African ancestry to date, said lead author Steven Micheletti, a population geneticist at 23andMe. The study's large sample size and geographic representation of Africa and the Americas, and its collaboration with historians and experts in African American studies, he said, are unique factors that made interpretations of the results more accurate and complete compared to preceding studies.

Joseph Graves, professor of biological sciences at North Carolina A&T State University who has studied the biological concept of race and its relation to social definitions of race, agreed that the study is broad and robust in re-examining and validating the events of the transatlantic slave trade, the distribution of African ancestry in different areas of the Western Hemisphere, as well as how these findings remained consistent with the historical narrative. "Based upon what I've seen, this is the most comprehensive study to date," said Graves, who was not involved in the publication. He added that he has seen no other genomic analyses better than this study.

"Why we wanted to do this was to actually give back to the study participants of African descent," said Micheletti. "For millions of people in the Americas, the story of the transatlantic slave trade is in part a story of their ancestral origins, so we wanted to better help them not only understand the history, but the genetic landscape because again, it is the story of their origins in many cases."

Analyzing genotyping data from over 50,000 participants between the ages of 18 and 80 years and documentation on over 36,000 voyages from the Slave Voyages database, the researchers revealed that "genetic evidence of Atlantic ancestry across the Americas is consistent with historical documents of the transatlantic shipping of enslaved Africans." A majority of the participants came from consented 23andMe customers, who sent in their saliva samples for DNA analysis. Customers who wanted to be involved in this research were given the option to fill out web-based questionnaires about their ancestry, ethnicity, and the birth locations of their four grandparents. Other data were obtained from the 1000 Genomes Project, the Human Genome Diversity Project, Angola, and previous studies of the Democratic Republic of the Congo, Sierra Leone, and Khoe-San speaking people.

"For many years, the world has understood the slave trade only through stories, documents, and these articles of the past," said Micheletti. "So, with global representation, study participants from across the Atlantic, we wanted to essentially say 'how well do these genetic connections between people in the Americas and people of Africa match historical records?' And in general, they match the records pretty closely, but there are a lot of deviations as well." 

Scientists at the National Genetics Institute, a subsidiary of LabCorp, extracted and genotyped the DNA from saliva samples of 23andMe customers and the Human Genome Diversity Project. The population structure of Atlantic Africa, plus the Americas, was assessed using the ADMIXTURE tool and ordination techniques, such as principal component analysis and uniform manifold approximation and projection. These techniques were not responsible for the main conclusions of the study but were used to determine the genetic differentiation within Africa and across the Americas, said Micheletti. The central drivers of the study's conclusions were the genetic connections between people of African descent in the Americas and the people of Africa, which were obtained from estimating identity by descent (IBD) within and between people representing regions in the Americas, Atlantic Africa, and western Europe, using the software package refinedIBD, along with 23andMe's ancestry composition analyses to identify the ancestral origins of chromosomal segments in individuals.

The researchers found that most Americans of African descent had roots in Angola and the Democratic Republic of the Congo and that these genetic contributions matched the historical records. They expected to find these connections because these areas of Africa were more strongly impacted by the slave trade, with over 5 million people being forcibly deported out of those regions to the Americas, said Micheletti. However, there was a discordance between the genetic data and historical records for some Atlantic African regions, he added. To solve some of these inconsistencies, the research team consulted with historians and looked at literature that had compiled historical diaries and newspaper articles.

One example of this discordance was the overrepresentation of Nigerian ancestry in the US and Latin America, which the scientists could explain by the intra-American trade of enslaved people from the British Caribbean. 

"We found this deviation of expectations in the United States-based African Americans, where they have a strong connection to the present-day region around Nigeria in Africa," said Micheletti. "There were actually very few enslaved people that were transported directly from the region of Nigeria to the United States. And to explain this, the intra-American slave trade actually has documents of many enslaved people being transported from the British-occupied Caribbean, so places like the Bahamas and Jamaica, to the United States."

The study also exposed the genetic underrepresentation of Senegambian ancestry across the Americas, which was also unexpected based on historical records of slave trading. The researchers found evidence of high mortality among slaves from the Senegambian region. Documents suggested that this was caused by the difficult working conditions of Senegambian slaves working in rice fields. Some people could have died from drowning or from animal attacks within these fields, like venomous snakes, said Micheletti, but malaria and poor working conditions in these muddy swamps were the likely main cause of increased deaths. 

"This was one case where we then looked at historical documents and found that Senegambians tended to be rice cultivators, they tended to be experts at growing rice in Africa," said Micheletti. "When they ended up in the United States, slavers typically put them on rice plantations that had a lot of standing water and were generally rampant with malaria."

There was also a higher contribution from African women to the gene pool in many parts of the Americas, which the researchers attributed to sexual exploitation of African women, higher mortality of enslaved men, and attempts to marry African women to European immigrants. This sex bias was stronger in Latin America than in the United States, which was also unforeseen, said Micheletti. He and his colleagues think this happened because lighter-skinned European men were encouraged to marry and reproduce with African females to produce lighter-skinned children and dilute African ancestry.

An additional reason for the heightened sex bias is the high mortality of enslaved men in Latin America, who therefore did not have a chance to have children. More than 60 percent of enslaved people, brought to each region of the Americas, were men, according to the study. About 9.1 million enslaved people were forced into Latin America, said Micheletti, whereas 400,000 slaves were brought directly to the US from Africa. With the abundance of male slaves in Latin America and the numerous opportunities to buy and replace slaves, slave owners did not care about maintaining their workforce, he said.

Another interesting finding Micheletti pointed out was that the study's genetic data supports the historical evidence that Latin America and the rest of the Americas practiced two distinct forms of racism. He said that Latin America focused on dilution of African ancestry by encouraging light-skinned Europeans to reproduce with African females, whereas other countries, such as the US, focused on segregation.

The team would like to further expand its studies by looking at the slave trade on the eastern coast of Africa, once it has study participants from these regions, said Micheletti. Currently, he and his colleagues are thinking about incorporating their recent research findings into a feature for 23andMe customers of African descent who want to track their ancestral origins from the transatlantic slave trade, he added.

Graves said these data present more evidence to people who are curious about the transatlantic slave trade but he does not think these findings will improve ancestry testing or predict genetic contributions to diseases within populations with African ancestry. He said the next steps could be to obtain ancient human samples from the time of the slave trade and compare their ancestry to the current populations.

"We always knew that particularly in the case of African Americans, that their ancestry came from Senegambia, Coastal Western Africa, Nigeria, and Central West Africa," said Graves. "What this study does is, it apportions the percentages of ancestry from those various areas, so it's a finer analysis of something we already knew."

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