Surprising genetic diversity of Roman Empire revealed

An international team of researchers led by Stanford University have used ancient DNA to map the migration of people during the Roman Empire. This new study published in the Journal eLife, took thousands of samples and discovered that at least 7% of the historic individuals carry ancestry uncommon in the region where they were sampled, implying that they were buried somewhere different to where they originated.

Will - Shivani, like last month's story on the origins of multiple sclerosis, is this another classic case of bone sampling?

Shivani - Yes, it is. Essentially, they took thousands of skeletons, took samples and sequence the DNA from them. These skeletons are between the first and seventh century, which is during the Roman Empire.

Will - And Aylwyn, to bring you in. What were the surprise findings of the paper? Was it just that we didn't realise how much people moved such a long time ago?

Aylwyn -
Well, that seems to be something that they found and, in a previous paper where they looked at the city of Rome, they found a lot of people there who clearly had come, or whose ancestry was from lots of different parts of Europe, of the Roman world. That's maybe not so surprising because Rome was the capital of the empire. People coming or being brought there from all over the world. But this paper then shows that actually that was true when you go out into the regions, into the provinces, and into the countryside and elsewhere around the Roman Empire, that you see similar levels of diversity. And you're right, maybe it's not unexpected because we know that empires do that they move people around. But you know, we can't assume these things are going to necessarily be found in the data and sometimes what we found in the past.

Shivani - What was actually quite interesting about the paper was, despite all of this migration, even today a lot of the populations are still quite genetically distinct, which is almost the opposite of what you would expect if there was such mass migration during the Roman Empire. The paper hypothesises there's two key reasons for that. The first is this idea of transient migration, which basically says that someone might be born in X and they might travel to Y for employment or due to military reasons. But they will only move to Y for a few months or a few years and actually never properly settle down there or reproduce there and returned back to their original place of birth. And that's something that haven't really happened in the history of human migration because generally when people moved, they moved there for good. And you know, part of the Roman Empire was creating huge networks of roads and waterways and that kind of allowed for this unique concept of transient migration. The second reason that they hypothesise might be why populations are still so genetically distinct is that after the fall of the Roman Empire, all these kind of global cities and transport links crumbled and sort of fell away. And so people generally then returned back to where they came from because there was no employment or military recruitment. So I think in the paper those were the two main reasons they attributed it to.

Will - Perhaps me and my jaded brain thought that migration is a fairly modern affectation because it's so simple now. And as you say, you'd assume back then if you moved it was once and permanently, but people were so much more transient than we realised and this genetics backs that up.

Aylwyn - Yeah, I mean it's very interesting how early this signal seems to set in. The signal being one where the level of diversity stops decreasing. Put it that way. That's what we see, as Shivani said, if you look at earlier periods, as we come forwards in time gradually people are, around Europe anyway, are becoming genetically more similar. There's mixing of all the different ancestries that we have at the very earliest prehistoric period. And then from the round about the Bronze Age onwards and into Roman time and up to the present day, the sort of the level of genetic diversity of heterogeneity, that stops decreasing. And certainly, one of the theories that this paper puts forward is that yes, we're finding people in other parts of the world, but they haven't necessarily produced and therefore their ancestry hasn't started to merge into the local population there. There is another possibility of course, which is that it's also possible you could imagine a society where it's just incredibly stratified and caste ridden, say, and therefore people just don't mate with people from different ancestral backgrounds which then subsequently become just different groups of people living in the same place. That would also have a similar effect of maintaining this kind of diversity of ancestry over a long period of time in different parts of the empire.

Will - Yes, I suppose that does bring the kind of uncomfortable nature of asking the question, why were these people moving in the first place? And a lot of it will be for jobs or for stuff like that, but many of them will have been moved against their will and perhaps the mating opportunities for said people were severely limited.

Aylwyn - Exactly. And I think maybe in the future we'll start to really explore the genetic and ancestral consequences of things like that. And you know, with the idea being then that we can turn around and say, well actually maybe this factor has been more prevalent than this other one. Or maybe they are both important, that it's not just transient movement and people moving back, but there are also these kinds of segregating factors involved as well.

Will - That's fascinating that genetics can be used to understand the social proclivities of such an old civilization.

Aylwyn - Yeah so they made a prediction, which I always like when papers do that. They said, if that's the case, what we might expect then following the fall of the Roman Empire, if we do the same study over the next sort of the subsequent several hundred years, is that actually then we do see a sort of a decrease in this heterogeneity, a reduction diversity in people start to stay put, be unable to travel and therefore we would see things starting to merge together. And at the moment we don't have sufficient amounts of data from the following period, but that's going to come along very soon. And so it's going to be a very exciting time in the relationship between genetics and early and mediaeval history.