Ancient DNA from the teeth of 14th-century Ashkenazi Jews in Germany already included genetic variations common in modern Jews

They do. A stretch of genetic material called mitochondrial DNA is inherited only from mothers. Different people around the world today carry subtly different variations of it. One variant of mitochondrial DNA is found in 20% of modern Ashkenazi Jews and is nearly absent in non-Jewish populations. We identified it in 35% of the Erfurt individuals.

In other words, a third of the people we sampled from the graveyard descended, via their maternal line, from a single woman. That so many people share the same ancestral mother implies that the population must have been extremely small in the centuries before.

In the Erfurt individuals, we also found mutations common in Ashkenazi Jews today but extremely rare elsewhere, including 16 disease-causing mutations, one of them in the well-known BRCA1 gene. Another research group sequenced the genomes of six Ashkenazi Jews from 12th-century Norwich, England and identified other disease mutations that are also still seen in Ashkenazi genomes today.

What was most striking about the founder event was how strongly the Erfurt Jews were affected. We estimate that the degree of relatedness of modern Ashkenazi Jewish genomes to each other is about what would be expected if they descended from a population that had been persistently small throughout the second half of the Middle Ages. How small? We calculated that a core of only 1,000-2,000 reproducing people during this time would be responsible for most descendants today.

When we repeated a similar calculation using the Erfurt data, we encountered a surprise. Based on the medieval DNA, our estimate of the size of the founding population was about 3-fold smaller, only around 500 people.

How could it be that we were detecting the same founder event – responsible for the same disease-causing mutations in the Erfurt and in the modern Ashkenazi Jewish communities – and yet its impact on the Erfurt Jews was larger?

To address that, we proposed there were additional medieval Ashkenazi communities that inherited much less DNA from the core group of reproducing people we identified for Erfurt. We don’t yet know who these communities were, but our modeling suggests that they must have existed and later mixed with Erfurt-like communities, averaging together to form today’s Ashkenazi Jews.

So sometime after the 14th century, genetic barriers between Ashkenazi Jewish communities must have broken down, and the archipelago of scattered early Ashkenazi Jewish populations collapsed into the homogeneous group seen today. This was accompanied by extremely rapid population growth, which then continued for centuries. The Ashkenazi Jewish community, which had originally been demographically peripheral in the Jewish world, with its center of gravity around the Mediterranean and the Middle East, eventually became the largest world population of Jews.

Erfurt and Norwich are just two locations. A richer picture of medieval Ashkenazi Jewish history will require sampling additional sites. How Ashkenazi Jews relate to Sephardi Jews and the many other living Jewish communities, and how all of these communities relate to Roman-period Judeans, are mysteries that ancient DNA research may also one day address. Any such research would need to take into account modern community sensitivities, and we think our work in Erfurt is a good model.

More broadly, this work provides a template for how ancient DNA, even from individuals who lived relatively recently, can reveal aspects of history that are otherwise invisible. By carrying out such studies, scholars can help reveal the roots of modern groups, enriching people’s understanding of themselves and each other.

This article is republished from The Conversation, an independent nonprofit news site dedicated to sharing ideas from academic experts. It was written by: Shai Carmi, Hebrew University of Jerusalem and David Reich, Harvard University. Like this article? subscribe to our weekly newsletter.

Shai Carmi received funding for this study from the Israel Science Foundation and the United States-Israel Binational Science Foundation. He is a paid consultant at MyHeritage.

David Reich receives funding for his research from the US National Institutes of Health, the Allen Discovery Center program (a Paul G. Allen Frontiers Group advised program of the Paul G. Allen Family Foundation), the John Templeton Foundation; a private gift from Jean-François Clin, and the Howard Hughes Medical Institute.