Tag Archives: slave trade

What DNA can tell us about the transatlantic slave trade in the Americas

Schematic shows general direction of the triangular trade routes between continents during the transatlantic slave trade. Credit: Micheletti et al./ The American Journal of Human Genetics.

Over 10 million African natives were forcefully taken as slaves and transported to the Americas to live the rest of their days as slaves. The same fate would befall their children for hundreds of years until the end of the 19th century. This dark and shameful episode in humanity’s history is etched in the genomes of people alive today in North, Central, and South America, as well as the Caribbean. In a new study, researchers tapped into this DNA to reveal new insights about the trans-Atlantic slave trade, some of which have scaped written records.

“Our study combined the genetic data of more than 50,000 people on both sides of the Atlantic with historical records of enslaved people to create one of the most comprehensive investigations of the transatlantic slave trade,” says first author Steven Micheletti, a population geneticist at 23andMe, who employed genetic data from the  Intra-American Slave Trade database. “One of the disturbing truths this research revealed was how the mistreatment of people with African ancestry shaped the current genetic landscape of African ancestry in the Americas.”

The researchers at 23andMe, a personal genomics and biotechnology company based in Sunnyvale, California, found that most African-Americans have their roots in Angola and the Democratic Republic of Congo. That was to be expected given historical records, but there were surprising patterns of migration and interbreeding that came to light when the researchers had a closer look.

One such finding was that Nigerian ancestry is over-represented in African Americans in the United States, mainly due to the “transport of slaves within the Americas, primarily from the Caribbean,” according to senior author Joanna Mountain, Senior Director of Research at 23andMe.

In contrast, the genetic flow between African Americans and Senegambians was much much lower than expected. During the height of the trans-continental slave trade, people from Senegal and Gambia disembarked in North America in great numbers.

But even this revelation wasn’t all that surprising when the researchers examined the historical records. Senegambians were mostly put to work on rice plantations in the US, which were infested with malaria and had very high mortality rates. As such, most died before they had the chance to pass on their genes.

The researchers were also able to infer the effects of policies enacted by slave-owners and local governments across the Americas. In some cases, the policies could be so different that the DNA differences became striking across many generations.

For instance, slave owners in the U.S. wanted enslaved Africans to have children with one another in order to boost the workforce. Even after slavery was abolished in the U.S. with the passing of the 13th Ammendment, those of African descent still found themselves segregated.

In contrast, slaves of African descent living in Latin America often interbred with white-skinned Europeans — in fact, the practice was very much encouraged in order to promote the “dilution” of the African ancestry. Today, the proportion of people with greater than 5% African ancestry is five times lower in Latin America than in the US, despite the fact that 70% of all African slaves disembarked in South America.

The Latin American dilution was mainly promoted between darker-skinned females and white-skinned Europeans — and this shows to this day in the DNA of their living descendants.

“Our analysis estimated about 15 African women had children for each African man in Central and South America, as well as the Latin Caribbean,” says Micheletti.

“The female bias is particularly shocking given that the majority of enslaved individuals were male,” he added, mentioning that female gene bias was also strong in North America.

Previously, another study published earlier this year by Brazilian researchers who studied the DNA of 6,267 individuals with more than 10% African ancestry from 25 populations came to similar conclusions. The study found that West Central African ancestry (from countries such as Nigeria and Ghana) is the most common in the Americas. West African ancestry (i.e. Senegal and Gambia) increases going northward while Bantu ancestry (from the south and southeast Africa) is more significant in the South of Brazil.

“The African Diaspora was so massive (>9 million people), that the genetic diversity observed in the African portions of our admixed genomes is similar to that of African populations of origin of slavery. However, admixture homogenized this diversity (and the mutations responsible for diseases) between the different populations of the African continent,” lead author Eduardo Tarazona-Santos, a researcher at the Federal University of Minas Gerais in Brazil and lead author of the study, told ZME Science.

These findings might not only provide new insights into the tragic slave-trade, but might also enable those of African descent to find their roots and come to a better understanding of what their ancestors had to go through.

“This paper conveys how the racist and dehumanizing acts endemic to the slave trade led to different patterns of African ancestry across the Americas that we can see in the DNA of people living today. 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,” says Micheletti.

The findings appeared in the American Journal of Human Genetics.

Researchers dive deep into the genetic legacy of the transatlantic slave trade

Print showing an alleged incident of an enslaved African girl whipped to death for refusing to dance naked on the deck of the slave ship Recovery, a slaver owned by Bristol merchants. Captain John Kimber was denounced before the House of Commons by William Wilberforce over the incident. In response to outrage by abolitionists, Captain Kimber was brought up on charges before the High Court of Admiralty in June 1792, but acquitted of all charges. Credit: United States Library of Congress’s Prints and Photographs division.  

Researchers in Brazil combined historical and genetic data to reveal new insights about the transatlantic slave trade that saw more than 9 million Africans shipped in chains to the Americans from the early 16th century until the mid-19th century. The findings suggest that the African populations imported their genetic diversity and spread their mutations in the Americas through admixture with indigenous and European populations.

“We know in the Americas that the slave trade was a human tragedy, but it is part of our history and identity. This is why my group, but mainly myself and my former PhD student Mateus Gouveia focused in the African Diaspora,” Eduardo Tarazona-Santos, a researcher at the Federal University of Minas Gerais in Brazil and lead author of the new study, told ZME Science.

African populations are the most diverse in the world, genetically speaking. Tarazona worked closely with colleagues in Brazil, Peru, and the United States to assemble what he calls the “largest up-to-date dataset of Americas and African genetic data”, which includes 6,267 individuals with more than 10% African ancestry from 25 populations.

Researchers compared the genetic data with historical demographic data from Slave Voyages database, which tracked and mapped the dispersal of enslaved Africa into the Americas.

“We came out with a mathematical method that makes this comparison compatible. Then we realized that comparing genetic and historical-demography data is something modern geneticists had forgotten to do during the last 10-20 years, but it this kind of comparisons were more common before and have a solid tradition in human population genetics, since the work by Luca Cavalli-Sforza (who passed away in 2018) sixty years ago in the Parma Valley in Italy, where he compared genetic data (from blood groups) with parish record data. So recovering this kind of work, is like making a tribute to Luca Cavalli-Sforza. Reading his books has been an inspiration for many young investigators that in the nineties decided to dedicate to human population genetics, as I did,” Tarazona said.

The Transatlantic Slave Trade transported more than 9 million Africans to the Americas between the early 16th and the mid-19th centuries. Credit: Eduardo Tarazona-Santos, of the Federal University of Minas Gerais in Brazil.

The researchers found that West Central African ancestry (from countries such as Nigeria and Ghana) is the most common in the Americas. West African ancestry (i.e. Senegal and Gambia) increases going northward while bantu ancestry (from south and southeast Africa) is more significant in the South of Brazil.

Historical records show that the transatlantic slave trade was at its height between 1750 and 1850. The new study found that this period also coincides with the most admixture between imported African populations and locals of European and indigenous ancestry. This timing implies that the 19th century was critical in shaping the structure of the African gene pool in the New World.

“The African Diaspora was so massive (>9 million people), that the genetic diversity observed in the African portions of our admixed genomes is similar to that of African populations of origin of slavery. However, admixture homogenized this diversity (and the mutations responsible for diseases) between the different populations of the African continent,” Tarazona told ZME.

All in all, the study provides unique insights into the gene flow caused by the massive transatlantic slave trade, whose influence is still important in today’s social and cultural setting in the Americas.

“Our results imply that the Africans imported most of their genetic diversity, including the mutations responsible for the diseases, and that admixture has spread these mutations in the Americas along most of the continent. In Africa, they are more compartmentalized geographically. This is important when we interpret data about where there are in the Americas mutations responsible for diseases such as cystic fibrosis and hereditary cancer,” Tarazona concluded.

The findings appeared on March 2 in the journal Molecular Biology and Evolution.

Scientists analyze 300 year old DNA from Caribbean slaves

Three hundred years ago, three African-born slaves from the Caribbean suffered a sad fate. No one knew who they are, no one knew what they went through, and until recently, no one knew where they came from. Now, researchers extracted and sequenced tiny bits of DNA to figure out where in Africa these people came from when they were captured and enslaved.

Image via MLK Task Force.

“Through the barbarism of the middle passage, millions of people were forcibly removed form Africa and brought to the Americas,” said Carlos Bustamante, one of the researchers, in a news release. “We have long sought to use DNA to understand who they were, where they came from and who, today, shares DNA with those people taken aboard the ships. This project has taught us that we cannot only get ancient DNA from tropical samples, but that we can reliably identify their ancestry. This is incredibly exciting to us and opens the door to reclaiming history that is of such importance.”

Indeed, it’s estimated that roughly 12 million people shipped from West and West Central Africa to the New World between 1500 and 1850, and very little else is known about the origin of these people. Hannes Schroeder, one of the study authors said:

“What’s new about our study is that we were able to obtain genome-wide data from really poorly-preserved skeletal remains using this new technique called whole genome capture. Those remains had essentially been lying on a Caribbean beach for hundreds of years so their preservation was really not good. But by enriching the poorly preserved DNA in those samples we were able to obtain enough data to be able to dig deeper into the genetic origins of those three individuals we analyzed.”

They used a technique called whole-genome capture to isolate enough ancient DNA to properly sequence and analyze – quite a challenging task, as the DNA was in a pretty bad state. They found that one of the former slaves had likely belonged to a Bantu-speaking group in northern Cameroon. The other two shared similarities with non-Bantu-speaking groups in present-day Nigeria and Ghana. All three of them were from between the ages of 25 to 40 years and lived sometime in the late 1600s.

“We were able to determine that, despite the fact that the three individuals were found at the same site, and may even have arrived on the same ship, they had genetic affinities to different populations within Africa,” said Maria Avila-Arcos, one of the researchers, in a news release. “They may have spoken different languages, making communication difficult. This makes us reflect on two things: the dynamics of the trans-Atlantic slave trade within Africa, and how this dramatic, ethnic mingling may have influenced communities and identities in the Americas.”

Journal Reference: Hannes Schroeder et al. Genome-wide ancestry of 17th-century enslaved Africans from the Caribbean. doi: 10.1073/pnas.1421784112