For over 1,000 years, the Roman Empire connected diverse populations in new ways through trade routes, economic and political cooperation, and joint military endeavors. Now, an international team led by researchers at Stanford Medicine has used ancient skeletal DNA to paint a detailed picture of travel and migration patterns during the empire’s peak.

Their study, published in eLife, analyzed the DNA of thousands of ancient humans, including 204 that had not been sequenced previously. It showed how mixed many areas of the Roman Empire were: at least 8% of individuals in the study did not originally come from the European, African or Asian region where they were buried.

Until now, we’ve had to rely on historical and archaeological records to try and reconstruct how populations interacted and changed during this time, says Dr. Jonathan Pritchard, professor of Genetics and Biology and a lead author of the study. Now we can add new details from a genetic perspective.

The team previously used ancient DNA to study genetic diversity in Rome and its environs over 12,000 years from the Stone Age to medieval times. They showed the area rapidly diversified around Rome’s official founding in 753 BC. They wondered how unique Rome’s diversity was as the empire’s capital, and how varied more remote areas may have been.

For this study, they focused on a narrower timeframe—from 3,000 years ago at the end of the Iron Age to today—but examined an area spanning the entire Roman Empire. They used existing DNA data from thousands of skeletons collected across the empire as well as Central Europe, Eastern Europe, Central Asia, Britain and Northern Europe, and Northern Africa. Additionally, they sequenced 204 new genomes from 53 archaeological sites in 18 countries, most from individuals who died during the Roman imperial and Late Antiquity periods between the 1st-7th centuries AD.

When we started this study, there weren’t many historical genomes from this period, so the new samples filled in gaps, said Clemens Weiss, a former postdoc in Pritchard’s lab who co-led the work and is now a research engineer at Stanford.

They first observed less diverse areas were often geographically isolated, like the mountainous Armenian highlands. However, most Roman Empire areas contained skeletons from varied genetic origins. Sardinia, the Balkans, and parts of Central and Western Europe showed especially high diversity.

In large part, the observations complement historian and archaeologist hypotheses, explains Margaret Antonio, a graduate student in Pritchard’s lab and co-author. For example, North African pottery was found across the Roman Empire. Now we also find genetic evidence of North African people residing in today’s Italy and Austria.

To better understand connected regions, they analyzed individuals buried in each location whose genetic ancestry did not match where they were found, suggesting they or recent ancestors traveled or migrated. Common patterns emerged among those not native to their burial locations. For example, British and Irish individuals were more likely to originate from Northern or Central Europe with far fewer from Southwest Europe or North Africa. The analysis helped explain how trade routes and military movements may have fueled diversity.

The new data led the researchers to a puzzling conundrum: if people had continued to move at the observed rate during the period studied, regional differences would have gradually begun to disappear. The genomes of the inhabitants of Eastern Europe, for example, would have become indistinguishable from those of Western Europe and North Africa, and vice versa. However, most of these populations – even today – remain genetically distinct.

This may be due, in part, to the fact that individuals did not always reproduce in the places where they died, and some may have traveled during their lifetime but returned home before having offspring.

The mobility of individuals declined dramatically when the Roman Empire collapsed. They do not have enough data from that period to know for sure, but hope to conduct future studies focusing on medieval times, the Enlightenment, and the Industrial Revolution to see how mobility patterns compare.

The researchers were excited to learn about Roman Empire population mobility. It shows movement is not new; Roman Empire peoples traveled very similarly to how we do today, says Antonio. They moved to trade and work. Some settled where they relocated, while others did not.


Sources

Sarah C.P. Williams, Researchers use ancient DNA to map migration during the Roman Empire (Stanford Medicine) | Margaret L Antonio, Clemens L Weiß, Ziyue Gao, et al., Stable population structure in Europe since the Iron Age, despite high mobility. eLife 13:e79714. doi.org/10.7554/eLife.79714


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