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Dec 28, 2023

DNA of Medieval Skeletons in Germany Sheds Light on Origins of Ashkenazi Jews

Rare opportunity to analyze genomes of 14th century Jews shows there were once

Rare opportunity to analyze genomes of 14th century Jews shows there were once two distinct populations of Ashkenazis. Also: the ‘Ashkenazi population bottleneck’ wasn't an episode, it lasted for 500 years

The origins of Ashkenazi Jews are shrouded in mystery. We know that the first Ashkenazi communities emerged in the Rhineland at the height of the Middle Ages, around the 10th century. But how and when Jews first reached the Rhine Valley, developing the distinct, rich culture that would eventually spread across Europe and much of the world , is not clear.

The origins of Ashkenazim have also been of interest to geneticists because they carry a disproportionate amount of gene mutations, some of which can cause chronic or fatal diseases.

Now a DNA study sheds new light on the roots of the Ashkenazi population and its early history, which turns out to have been more complex than we thought, says Prof. Shai Carmi, a geneticist from the Hebrew University in Jerusalem who led the research.

Carmi, his doctoral student Shamam Waldman, and an international team of researchers were able to extract DNA from the teeth of 14th century skeletons buried in the Jewish cemetery of Erfurt, in central Germany.

The study in itself is a rare feat. In the last decades, ancient DNA studies have been legion and revealed much information about the origins and movement of human populations in the past. But because disturbing human remains is a big no-no under Jewish religious law, it is not often that researchers get to extract DNA from the ancient bones of members of the tribe.

In fact, the new study, published Wednesday in the journal Cell, is one of the first two efforts to sequence ancient DNA from the remains of Ashkenazi Jews. In another such study, published in August, researchers extracted DNA from 17 bodies found buried in a medieval well in Norwich, only realizing these may have been the Jewish victims of a pogrom when they identified genetic mutations typical of Ashkenazim. In that case, the identification of the bodies as Jewish, while very likely, remains unconfirmed.

For the Erfurt study, there is little doubt of the identity of the subjects, as Carmi and colleagues obtained the DNA of 33 individuals buried in the town's medieval Jewish cemetery.

The Jewish community of Erfurt existed from the late 11th to the mid 15th century, with a short gap following a 1349 pogrom that all but wiped out the entire community.

After expelling the Jews in 1454, the city built a granary on top of their cemetery. Come the year 2013, when the granary was no longer in use, Erfurt converted it into a parking lot, which led to additional construction and an archaeological rescue excavation. Armed with recent rabbinical rulings that allow for ancient DNA research to be conducted on detached teeth, the researchers obtained permission from the local Jewish community to sample the remains, which were later reburied in the town's 19th century Jewish cemetery.

Radiocarbon showed that the bodies, all but one buried with their feet facing Jerusalem according to Jewish custom, dated to the 14th century, though it is unclear whether they lived before or after the 1349 massacre, one of many perpetrated against European Jews during the Black Death. There were 19 females and 14 males, many of them children, and only one individual showed signs of a violent death: several blows to the head with a sharp object.

An ancient split

The analysis of their genomes showed that at least eight of these Jews carried the same pathogenic gene variants typical of Ashkenazim today, which can cause severe illnesses. These include retinitis pigmentosa, which degrades the retina; Gaucher disease, which causes a dangerous buildup of fatty tissues in organs or bone tissue; Usher syndrome, which causes deafness and blindness, as well as the BRCA1 variant that increases the risk of breast and ovarian cancer.

The prevalence of so many specific mutations among Ashkenazi Jews has long been suggested as a sign that this group went through a bottleneck, or a "founder event." In other words, for whatever reason – all Ashkenazim today descend from a single tiny group.

"We don't know the nature of the bottleneck : whether it was a continuous decline over several centuries or a single event where, for example, a small group moved away from the main population, which is a classic founder event," Carmi says.

The upshot would have been that the members of this small group married within their small group, leading to diminished genetic variation. In extremes, loss of genetic variation and isolation lead to speciation; in the case of Ashkenazim, it meant that even as the population grew, its members shared a lot of genes, including "bad" ones that, when inherited from both parents, cause illness or even death.

Another sign that all Ashkenazim extant today descend from a tiny founding population is the fact that 40 percent of modern Ashkenazim carry the same four sequences of maternally-inherited mitochondrial DNA, meaning they descend from just four ancestral mothers. This was even more pronounced in the Erfurt Jews. More than a third of the individuals in the sample descended from a single woman through their mitochondrial DNA, the researchers report in Cell.

The singular finding of the Erfurt study is that while the DNA of modern Ashkenazim is fairly homogenous, in the Middle Ages this Jewish population could have been divided in two genetically distinct groups. One had greater Middle Eastern and Southern European ancestry and was genetically closer to modern Ashkenazim originating in France and Germany. The second had a similar ancestry mix with an additional genetic component typical of Eastern Europe, Carmi says.

Since both groups shared the typical "founder mutations," the most likely explanation for this difference is that they both descended from the same small original population, which then split into two. One settled (or remained) in the Rhineland and one headed for Eastern and Central Europe.

Then, at the end of the Middle Ages, in places like Erfurt, the two communities began to mix anew as a result of migrations, and eventually coalesced into today's homogenous Ashkenazi genome.

While it was known from historical records that Jews from Bohemia, Moravia and Silesia migrated westward into Erfurt at this time, it was not recognized that these two populations had been genetically distinct, Carmi says.

By the way, this Eastern European component should not be seized upon as a way to support the long-discredited "Khazar hypothesis" – that is, the claim that Ashkenazim have no link to the ancestral population of Judah but descend instead from the Khazars, an early medieval kingdom in the Caucasus where part of the population had converted to Judaism.

The Eastern European component found in some of the Erfurt Jews is only a minor fraction of their ancestry, and there were no major direct links to the Caucasus, Carmi tells Haaretz.

Italian ancestry, but what's an Italian?

The Erfurt study doesn't dispel all the mysteries surrounding the origins of Ashkenazim, but it does give us some hints. Firstly, it further confirms that the bottleneck started sometime in the early Middle Ages – certainly before the 14th century and apparently before the 12th century, the time of the putative Jews found in Norwich.

Carmi and colleagues also compared the DNA of the Erfurt Jews to modern genomes from around the Levant and the Mediterranean to figure out their ancestry. The most statistically probable model indicates that both groups of Erfurt Ashkenazim could trace 65 percent of their ancestry to southern Italy, 19 percent to the Levant and 16 percent to Eastern Europe.

The strong Southern Italian component versus the Levantine one may seem surprising. But it should be noted that previous research has shown that in antiquity, particularly during the heyday of the Roman Empire, there was a large population influx into Italy from the Near East, which injected a strong Levantine genetic component into the Italian genetic mix.

Carmi emphasizes that these numerical estimates may not be precise because they rely on comparison with the genomes of modern people, which may be genetically different from those living in the same regions hundreds of years earlier.

A very long bottleneck

Anyway, by comparing the DNA of the Erfurt Jews with that of modern Ashkenazim, Carmi's team created a model of the group's progression over the centuries. According to this model, the Ashkenazi population only began expanding 500-600 years ago, just after the time of the people buried in the Erfurt cemetery.

Before that, the large number of common ancestors that the DNA shows suggests that the bottleneck was a protracted event, which began more than 1,000 years ago and kept the Ashkenazi population small for centuries. Specifically, for nearly 20 generations, or around 500 years, the so called "effective population size," that is the number of people who reproduced in every generation, may have been only 1,000 to 2,000, this model says.

Interestingly, this range is based on the modern Ashkenazi population, but when the researchers calculated the effective population size for the Erfurt sample, they came up with a number three times lower, 300 to 700.

This means that the ancestors of the Erfurt Jews experienced a much longer or more severe bottleneck, notes Prof. David Reich, a geneticist at Harvard University and one of the other authors of the paper. It also suggests that back in the Middle Ages, there were even more Ashkenazi groups out there that haven't been identified yet and who must have had a shorter bottleneck. When they eventually mixed up with the rest of the Ashkenazim, they produced the average result we see in the modern population, Reich says.

"So what we are seeing is an archipelago of populations, who then start to coalesce into the homogenous Ashkenazi population that we know today," he says.

While each isolated group may have experienced the bottleneck differently, it does look like the number of Ashkenazim stayed very low for a very long time.

"There is evidence that the bottleneck was ongoing for several generations, starting around the year 800-900," Carmi says.

While not providing conclusive evidence, all of this is consistent with a scenario in which the founders of the Ashkenazim were living within a larger community, likely in Italy or somewhere else in Southern Europe, and then left to establish their own communities in Northern Europe.

"To remain culturally distinct they had to marry within the community, so the population remained small for a long time," Carmi concludes.

How and why the initial migration north happened remains obscure, as does the subsequent split between the two distinct Ashkenazi groups identified at Erfurt.

Of course, Carmi cautions that his team's study is based on the DNA extracted from people who lived in a specific period in a single medieval town. Future studies, particularly from older Ashkenazi burials across Europe, may reveal further twists and turns in this complex chapter of Jewish history, he says.