The transatlantic slave trade's genetic legacy

Let’s jump forward a hundred thousand years to what might be the largest movement of people in recorded history: the transatlantic slave trade. For hundreds of years, people were taken from Africa across to the Americas, leaving an enormous legacy on our world today. The slavers kept thorough shipping records, but for historians, it’s always unclear whether they’re seeing the full picture. That’s why geneticists from the company 23andMe have been poring through the genes of people in the Americas today, and checking to see if their genetic ancestry to parts of Africa matches the historical records. Steven Micheletti from the team explained their findings to Phil Sansom...

Steven - Billions of people have been directly moved by slave trades over the course of human history. But the largest slave trade in recent history - the transatlantic slave trade - was this triangular trade between Europe, Africa and the Americas where one of the commodities happened to be human lives. During this event between the 16th and 19th centuries, historians estimate about 12.5 million Africans were taken and forced across the Atlantic. But it's important to note that only about 10 million of these enslaved people survived the voyages.

Phil - 10 million people is a lot to have on your plate still though. So how did you go about looking into this?

Steven - Our team started by reviewing the history of the transatlantic slave trade through studying slavevoyages.org, which is this digital compilation of about 35,000 transatlantic shipping manifests. And so for economic purposes there were these detailed records kept which documented each enslaved person taken from Africa. So we looked at those records, and such variables as the number of individuals taken from each region of Africa, and where in the Americas these individuals were being taken to, and other variables like men, women, and children being taken. And so that's on the historical record side. But we're of course experts in genetics. So we wanted to actually look at the genetics of people across the Americas, and we're fortunate to have 50,000 study participants across the world today, willing to share their DNA for this research. We specifically looked at genetic data from these participants today who have African ancestry, and we traced their DNA back to populations across Africa to see if their genetic connections match the estimates from shipping manifests. Just to give a quick example, if more people were taken from the Congo region of Africa and brought to say, Brazil, does this mean that Afro-Brazilians today have strong genetic connections to populations of the Congo?

Phil - Broadly then, when you're looking at these two lines of evidence, do they seem to match up?

Steven - Broadly? Yes. In most regions of the Americas, there was a strong correlation between the strength of genetic connection with an African population and the number of individuals taken from that population during the slave trade. For instance, the majority of enslaved people forced into parts of the Caribbean were from tribes of Nigeria and consequently people in the Caribbean today tend to have the strongest genetic connections with tribes of Nigeria. And that's common in many of the countries we looked at, but it wasn't always the case.

Phil - I mean, you emphasised the word broadly when you started that answer, which makes me think that there are some places where you found little mismatches. Is that true?

Steven - Yes. And there's, there's definitely a lot of discordances that we came across. And let me give you examples of two broad disagreements. One is, by far the most enslaved people were taken to Latin American countries. Compare 400,000 enslaved people that were brought directly to the US from Africa versus about 4 million that were brought directly to Brazil. And our expectation then is because so many Africans were brought to Latin America, there should be more African representation there. However, we found the opposite to be true. People of African descent from Latin American countries tended to have the least amount of African ancestry. So that was a big surprise.

And then the second broad disagreement was that mostly men, according to the records, were enslaved during the transatlantic slave trade. But we have genetic evidence that people of African descent tend to have inherited their African ancestry from their maternal side. So in other words, African women were estimated to have been reproducing more than African men.

Phil - Have you got any way to explain that? Because that seems like a weird couple of blips to have in your data.

Steven - For one, we found that there were different national ideologies and different ways of treating enslaved people between Latin American countries and other countries. Latin American countries tended to work their enslaved people to death. And because of this, enslaved people weren't having many children and weren't able to pass on their African ancestry. And therefore there's not a lot of African representation today. In terms of the African women reproducing at higher rates, this matches up with the known accounts of rape and exploitation of African women over time, where basically African women were forced against their will to reproduce.