Something Happened to Ashkenazi Jews in Medieval Europe to Eliminate Genetic Diversity, Study Finds

Something Happened to Ashkenazi Jews in Medieval Europe to Eliminate Genetic Diversity, Study Finds

About two-thirds of Jews today – or about 10 million people – are Ashkenazim, referring to a recent Central and Eastern European origin. They primarily reside in the United States and Israel. Ashkenazi Jews carry a particularly high burden of disease-causing genetic mutations, such as those in the BRCA1 gene associated with an increased risk of breast and ovarian cancer.

This genetic burden suggests that the population was shaped by what geneticists call a founding event or bottleneck. In other words, a small number of forbears and ancestors have contributed a large part of the modern gene pool. As the population grew and the descendants of these founders had many children, disease mutations carried by the few founders became widespread.

One of the most striking characteristics of Ashkenazi Jews today is their genetic homogeneity, with almost no discernible difference in ancestry between Ashkenazi Jews around the world. Were Ashkenazi Jews also similar to each other in the past? What were their origins? To what extent was the gene pool shaped by intermarriage with non-Jews?

New technology has made economical sequencing of whole genomes from skeletal remains practical. Weand 30 colleagues, mostly from Israel, Germany and the United States, investigated these questions by sequencing the centuries-old remains of Ashkenazi Jews from the medieval Jewish community of Erfurt, Germany.

DNA sequencing of a medieval cemetery

Previous studies of the genomes of Ashkenazi Jews living today have clearly shown that the founding event occurred in medieval times. But the earlier geographic origins of Ashkenazi ancestors are poorly understood.

The earliest historical records of Ashkenazi Jews come from the Rhineland in West Germany in the 10th century. In the hundreds of years that followed, an increasing proportion lived in Eastern Europe. Despite periodic persecution, the number of Ashkenazi Jews grew and peaked at over 10 million in the mid-20th century, before an estimated six million Jews were murdered in the Holocaust.

The medieval Ashkenazi Jewish community of Erfurt, Germany, existed between the late 11th century and the mid-15th century. After a vacuum following a massacre in 1349, Erfurt’s Jewish community became one of the largest in Germany – in fact, one of the oldest intact Jewish synagogues in Central Europe is in Erfurt – but the Jews were expelled in 1454. After that, the city built a granary above the Jewish cemetery.

In 2013, the attic was transformed into a parking lot. Prior to construction, the state conducted a salvage dig that uncovered 47 graves, most of which were sampled for DNA before the skeletons were reburied in the 19th-century Jewish cemetery.

Our study required the examination of the local Jewish community, as traditional Jewish law prohibits disturbing the dead under most circumstances. But a recent rabbinical study has suggested that research into ancient DNA is permitted if scientists use loose teeth from previously excavated remains. We followed this approach in order to be sensitive to community perspectives.

Today’s population is a mixture of past groups

We sequenced 33 individuals who lived in the 14th century. Among them were two families: a mother and her two children, and a father, presumably slain with a sword to the head, and his daughter.

Our first question was: do medieval Erfurt Jews and modern Ashkenazi Jews belong to the same genetic population? On average, yes. There has been virtually no incorporation of genes from non-Jewish European populations over the past 600 years.

But the biggest surprise was that the Jews of Erfurt were significantly more diverse than modern Ashkenazi Jews.

Some medieval individuals had greater Middle Eastern ancestry; they were genetically most similar to modern Ashkenazi Jews from France and Germany.

Others had greater Eastern European ancestry, consistent with historical evidence that a number of people living in Erfurt between 1350 and 1400 had surnames indicating origins in the East, as well as Slavic names.

The two groups – those of more Middle Eastern or more Slavic origin – also had distinct levels of oxygen isotopes in their teeth, indicating that they used different sources of water in childhood, and therefore, at least one of the groups must have included migrants.

Nevertheless, individuals from both groups were buried side by side, suggesting no social segregation.

Non-genetic research has suggested that in the Middle Ages Ashkenazi Jews were culturally divided into two major groups. Western Jews lived in the Rhineland, where Ashkenazi Jews first settled. They may correspond to the Erfurt group with the greatest Middle Eastern ancestry. Eastern Jews, from East Germany, Austria, Bohemia, Moravia and Silesia, may correspond to the Erfurt group with the greatest Eastern European ancestry.

Erfurt was on the geographical boundary between the two medieval Jewish communities, and by the 14th century was probably a home for Jews belonging to both. This may explain our detection of two genetically distinguishable clusters in one location.

Modern Ashkenazi Jews do not exhibit medieval genetic heterogeneity. Instead, their genomes look like an almost equal mix of the two Erfurt groups. Our genetic results are consistent with studies of names, dialects and religious rites, which suggest that Western and Eastern groups eventually merged and formed a single Ashkenazi culture.

A founding event left its genetic mark

Our next question was whether the Jews of Erfurt showed signs of the founding event so evident in the genes of modern Ashkenazi Jews.

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

In other words, one-third of the people we sampled at the cemetery 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 previous centuries.

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

What was most striking about the founding event was how deeply the Jews of Erfurt were affected. We estimate that the degree of relatedness of modern Ashkenazi Jewish genomes to each other is roughly what one would expect if they were descended from a population that had been consistently small throughout the latter half of the Middle Ages. How small? We calculated that a nucleus of only 1,000 to 2,000 individuals reproducing at that time would be responsible for most of the offspring today.

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

How is it that we detect the same founding event – ​​responsible for the same pathogenic mutations in Erfurt and in modern Ashkenazi Jewish communities – and yet its impact on the Jews of Erfurt was greater?

To solve this problem, we have proposed that there are other medieval Ashkenazi communities that inherit much less DNA from the core group of breeding people that we have identified for Erfurt. We don’t yet know who these communities were, but our modeling suggests that they must have existed and then mixed with Erfurt-like communities, coming together on average to form today’s Ashkenazi Jews.

Thus, sometime after the 14th century, the genetic barriers between Ashkenazi Jewish communities must have broken down, and the archipelago of early dispersed 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 was originally demographically peripheral in the Jewish world, with its center of gravity around the Mediterranean and the Middle East, eventually became the world’s largest population of Jews.

A model for future studies

Erfurt and Norwich are just two places. A richer picture of medieval Ashkenazi Jewish history will require sampling additional sites. The relationship between Ashkenazi Jews and Sephardi Jews and the many other living Jewish communities, and the relationship of all of these communities to Roman-era Judeans, are mysteries that ancient DNA research may also one day solve. . Any such research should take into account the sensitivities of the modern community, and we believe our work in Erfurt is a good model.

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

This article is republished from The Conversation under a Creative Commons license. Read the original article here.

(The Conversation is an independent, nonprofit source of news, analysis, and commentary from academic experts.)

#Happened #Ashkenazi #Jews #Medieval #Europe #Eliminate #Genetic #Diversity #Study #Finds

Leave a Comment

Your email address will not be published. Required fields are marked *