The slave trade's genetic impact

We're discussing history’s largest ever forced migration, the transatlantic slave trade: hundreds of years of ships stealing people from Africa across to the Americas. Steven Micheletti, a geneticist who works for the company 23andMe, has been part of a team looking for hints of this enormous upheaval in the genes of people in the Americas today, and checking to see if records of the slave trade really paint the full picture; as he told Phil Sansom...

Steven - To study a huge impactful event like this you really need global representation of study participants. And so fortunately we have a huge database of individuals that want to be part of our research.

Phil - How many people have you got in this study then?

Steven - In this study we have just over 50,000 individuals.

Phil - And that adds up to how many genes, or how many bits of DNA? Just so I can get it in my head...

Steven - In general we're looking at around a million different genetic variants.

Phil - Now what does that tell you? Because this is just the genes that people have today.

Steven - Right, but people today inherit those genes from their ancestors that existed long ago. So that's how we make the connection: we're essentially looking at the common ancestors between people that are currently in Africa and people that are currently in the Americas. One of the largest analyses that we focused on is called 'identity by descent', and like the name implies we're looking at identical sections of your genome that are shared with somebody else. And the reason those segments are shared with another person is because you have a common ancestor.

Phil - And can you be absolutely sure that it's not just a coincidence, you've ended up with the same genes by accident?

Steven - Yes, we do have different criteria that we apply to these analyses to give us a lot of confidence. For instance, we're looking at basically portions of the genome that are at least 3 million base pairs long, which is a large chunk of the genome.

Phil - Okay, that'd be a pretty big coincidence if you both had that same 3 million chunk by accident!

Steven - Absolutely.

Phil - You also have genes from people in Africa today that you're comparing these sequences to?

Steven - Correct. We have people in Africa that associate with over 24 different ethnolinguistic groups along the Atlantic coast of Africa; and these groups were heavily impacted in the past by the slave trade.

Phil - Right, so you've got then 50,000 people's gene, and you're finding the ancestors that are in common that may have come over on the slave trade. But these are untold millions of people transported against their will; how many actual common ancestors could you get with 50,000 people's samples?

Steven - Yeah, as you point out, there were so many people impacted by the slave trade; but people would have had to have reproduced for there to be any genetics to compare whatsoever. And what we're finding in this study is that there's cases where enslaved people simply weren't reproducing, and we're probably not able to capture that signal.

Phil - How many ancestors have you got then?

Steven - Quite a bit. The vast majority of people in the Americas have a genetic connection back to Africa. And one of the unique things about the transatlantic slave trade is that it was pretty well documented. Each shipping voyage from Africa to the Americas has shipping manifests associated with it, so we're actually able to compare these genetic results with the historical shipping estimates.

Phil - And what did you find?

Steven - In large, the shipping records match the genetic results. For instance, in the Congo region of Africa about 5.7 million people were enslaved, and we find that the most genetic connections are with the Congo region. However, that's not the full story; when we start to look in more detail at certain regions of the Americas we do find discordances between the shipping records and the genetic results. For instance, in the United States we tend to see an overrepresentation of genetic connections to Nigeria, and an underrepresentation of genetic connections to Senegal and the Gambia.

Phil - Why?

Steven - The overrepresentation of Nigerian ancestry, we believe to be caused by these other slave trades that were going on simultaneously with the transatlantic slave trade, generally forcing people out of the Caribbean into different regions of the Americas. And what we believed happened is that people with Nigerian ancestry in the Caribbean were being forced into the United States. Another piece of literature that supports this is that people from present day Nigeria were often forced into these breeding programs in the United States.

Phil - Oh, dear...

Steven - And there's evidence that this typically happened more in people with Nigerian ancestry.

Phil - Wow. Now you're getting this data from people who sign up to your database. Are these people who want to know the results of what you found, who might be interested; or are those people who don't want to know; or is it by necessity anonymous?

Steven - By necessity it is anonymous. One of the goals of this study though was to give back to people who might be interested in this topic. So we're hoping that study participants will read this and gain further insight into the slave trade. There might be personalised results that come out of this study as well.