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

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

Some medieval individuals had greater Middle Eastern ancestry; they were genetically most similar to modern Ashkenazi Jews with origins in 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 given names.

The two groups – those with more Middle Eastern or more Slavic origins – also had distinct levels of oxygen isotopes in their teeth, indicating they used different water sources in childhood, and thus, 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 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 greater Middle Eastern ancestry. Eastern Jews, from eastern Germany, Austria, Bohemia, Moravia and Silesia, may correspond to the Erfurt group with the greater Eastern European ancestry.

Erfurt was at the geographic boundary between the two medieval Jewish communities, and in the 14th century, it was likely a home to Jews belonging to both. This may explain our detection of two genetically distinguishable groups in one place.

Modern Ashkenazi Jews don’t show the medieval genetic heterogeneity. Instead, their genomes look like a nearly even mixture of the two Erfurt groups. Our genetic results fit with studies of names, dialects and religious rites, which suggest that the Western and Eastern groups eventually merged and formed a single Ashkenazi culture.

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

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