Ashkenazi Jews Have Become More Genetically Similar Over Time

A study of skeletons unearthed from a medieval Jewish cemetery in Germany has revealed a surprising genetic split among Ashkenazi Jews of the Middle Ages that no longer exists.

The analysis, the first of its kind from a Jewish burial ground and the product of yearslong negotiations among scientists, historians and religious leaders, shows that Ashkenazim have become more genetically similar over the past seven centuries. Two Jews walking the cobblestone streets of 14th-century Germany were more genetically distinct, on average, than any two Ashkenazi Jews alive today.

“That is wild!” said Dr. Harry Ostrer, a medical geneticist at the Albert Einstein College of Medicine in the Bronx and a co-author of the new study. “Despite the rapid growth of the Ashkenazi Jewish population during the last 700 years, the population became more homogeneous.”

The study, published on Wednesday in the journal Cell, compared DNA extracted from the teeth of 33 men, women and children buried in the cemetery with DNA taken from hundreds of modern Jews from around the world. Previous studies have shown that modern communities are a genetic mélange, with Ashkenazim the world over carrying essentially the same collection of DNA sequences.

But the medieval remains tell a different story. They show that European Jews at the time came from two divergent gene pools.

Each group shared the same genetic ancestry, dating back to a small founder population that most likely emigrated from Southern Europe and reached the German Rhineland at the turn of the first millennium. But the DNA analysis also revealed a genetic divide among the skeletons, which could have several explanations. In one scenario, both groups originated from the Rhineland. One branch then stuck around the region, while the other headed east to modern-day Poland, Czech Republic, Austria and eastern Germany.

Alternatively, Eastern Europe might have been settled by a different population of Jews who then mixed to a limited extent with their Jewish neighbors to the west.

Either way, the two groups remained fairly isolated from each other for generations, as evidenced by their discrete genetic lineages. Then, prompted by massacres, expulsions and economic opportunities, they reunited in places like Erfurt, the central German city that ishome to the cemetery where the remains were disinterred.

“It’s a supercool study,” said Itsik Pe’er, a computational geneticist at Columbia University who was not involved in the research. “Ancient DNA sequencing is a cheat-code that can take you to places where you don’t have information today.”

The existence of an east-meets-west community in Erfurt is also supported by the historical record, which includes detailed accounts of a violent pogrom on March 21, 1349 — a Saturday. Angry mobs entered the local synagogue and attacked Jews in the midst of prayer. Few, if any, survived.

After the massacre, Erfurt’s leaders took possession of property and belongings. They even collected on debts owed to the murdered Jews. But just five years later, the need for lost tax revenue prompted the city to invite Jews back.

They came from far and wide. Tax records show names denoting origins from all over Europe — including some from distant cities that had experienced their own antisemitic upheavals. “In the middle of the German-speaking lands, thiswas the place to be at the time,” said Maria Stürzebecher, a medievalist who is the curator of the Old Synagogue Museum in Erfurt. At least, that is, until 1453, when Jews were forced out again.

The same migration patterns could be seen in the excavated teeth.

Isotope readings from the dental enamel showed that many people were migrants who had grown up elsewhere. But the DNA took this finding one step further, showing that Erfurtian Jews came from multiple places, and that those populations were genetically distinct.

“This evidence both raises new questions and confirms stories we’ve been telling for a long time,” said Elisheva Baumgarten, a social historian at the Hebrew University of Jerusalem who was not involved in the study.

Preserved documents on money-lending practices show that the Jews from each subgroup largely formed business alliances with members of their own kind, according to Maike Lämmerhirt, a historian at the University of Erfurt and a co-author of the study. But both groups prayed in the same synagogue. They all cleansed in the same ritual bath. And, ultimately, they all lay side by side in the same cemetery.

The Erfurt skeletons carried many of the same disease-causing gene mutations that Ashkenazi Jews worry about today. That suggests a population bottleneck must have occurred before the Erfurtians were born — one in which small numbers of individuals seeded an entire population, leading to genetic similarities and the amplification of certain gene variants.

Scientists had previously calculated that the bottleneck event of the Ashkenazi Jewish population occurred roughly 600 to 800 years ago. But the new study, along with a British study published this year that examined six 12th-century skeletons found in England, suggest it could have been even further back.

“Given the date of these samples, we’re putting it really at the very, very old end of those estimates,” said Mark Thomas, an evolutionary geneticist at University College London who led the British study.

“If you put the two papers together, they’re completely in agreement — which is pretty cool,” said Ron Pinhasi, an anthropologist and geneticist at the University of Vienna in Austria who was not involved in either study.

Rabbinic law generally frowns on exhumation of corpses, out of concern for the dignity of the dead. Scientists therefore cannot excavate Jewish gravesites purely out of academic intrigue.

But what happened in Erfurt had nothing to do with the scientists.

In 2013, a storehouse that was built on top of the cemetery more than 500 years earlier was converted into a parking garage. Karin Sczech, an archaeologist then with the state preservation office, knew that the construction might disturb some ancient Jewish remains.

Dr.Sczech came to the work site a day before excavation was scheduled to begin, only to find that the contractor had already broken ground. Inside the bucket of an actively digging excavator were the bones of a small child.

“I yelled at the driver and said ‘stop,’” recalled Dr. Sczech, now a UNESCO-World Heritage coordinator for Erfurt.

She and her team discovered 47 graves in an area roughly the size of a volleyball court. In consultation with the local Jewish community, the archaeologists meticulously removed the skeletons and brought themback to the local archives.

There, the bones sat for many years. The plan had been to rebury the bodies quickly, once scientists had a chance to study the remains. But the anthropologist involved in the effort became tied up, causing a yearslong delay.

Lucky for genetic science that he did. Had the anthropologist been more prompt, the skeletons would have been back in the ground before the geneticists who led the new study, David Reich of Harvard and Shai Carmi of the Hebrew University of Jerusalem, ever knew about their existence.

The researchers set out in 2017 to find an ancient Jewish cemetery undergoing excavation, with the hope that they could take a small sample for genetic testing.

Dr. Carmi took the lead. He asked the advice of Ephraim Shoham-Steiner, a historian at Ben-Gurion University of the Negev, in Israel. “I said, ‘If there’s anywhere that this might actually come into play, it would be in Erfurt,’” Dr. Shoham-Steiner recalled.

At first, the top rabbi in Erfurt shot down the idea. There are situations that allow for DNA testing on Jewish corpses — for instance, families of Yemenite children who disappeared in the early years of Israel’s establishment can request graves be opened for forensic identification.

But the reasoning in those cases centered on concrete benefits to the deceased. Scientific research performed on anonymous bodies is different.

Dr. Carmi consulted a rabbinical court judge in Israel — Rabbi Ze’ev Litke, founder of the Simanim Institute in Jerusalem, which helps people determine whether they have Jewish ancestry through genetic testing — who ruled that it would be permissible to isolate DNA from teeth or tiny detached bones of the inner ear that, unlike the rest of a skeleton, do not require reburial under Jewish law.

Convinced by the argument, the rabbi in Erfurt changed his mind. The project was a go. Dr. Sczech found that 38 of the skeletons had at least one detached tooth.

Soon, Dr. Reich was flying back to Boston with zip-top bags full of medieval molars, bicuspids and incisors. Using techniques that won this year’s Nobel Prize for Medicine or Physiology, Dr. Reich and his colleagues successfully extracted DNA from 33 of the teeth.

The scientists hope their approach to community engagement will provide a road map for others hoping to examine the DNA of ancient remains, whether from Jewish cemeteries or otherwise. “This really is a sort of prototype for what can be done in similar studies,” Dr. Reich said.

Viewpoints differ among authorities on Jewish law, or Halakha, about whether procuring any DNA during an archaeological excavation of known Jews is above board.

Rabbi Myron Geller, a scholar of Jewish burial practices and a former member of the Committee on Jewish Law and Standards for the Conservative movement, described the rationale adopted by Rabbi Litke and the study authors as the “firmest halakhic perspective possible.”

But others questioned whether the abstract benefits of scientific knowledge were sufficient grounds to merit desecrating the dead. “It gives me pause,” said Rabbi Joseph Polak, chief justice of the Rabbinical Court of Massachusetts.

On a recent trip to Yad Vashem, the Holocaust memorial atop Mount Herzl in Jerusalem, Dr. Carmi strolled through the Valley of the Communities. In this massive monument to destroyed Jewish communities, he found the name Erfurt. Just as in medieval times, hundreds of Jewish residents of Erfurt were murdered during the Nazi era.

Standing there, Dr. Carmi reflected on the pieces of lost history that his genetic analysis had helped laid bare. “It was a great honor for me personally to bring their story to life,” he said.