A coat of many colors: medieval-DNA findings detect interwoven strands of Ashkenazi heritage
A coat of many colors: medieval-DNA findings detect interwoven strands of Ashkenazi heritage
Digging into the exciting new findings from early Jewish samples
Note: also see Ashkenazi Jewish genetics: a match made in the Mediterranean
As of 2020, Ashkenazim, or the Jews of Northern Europe, account for 75% of the world’s Jews. Today, after a century of scholarship, we’re getting close to finally understanding their origins, both culturally and genetically. The Ashkenazim are not simply the descendants of European gentile converts to Judaism. All carry a clear genetic signature of some Middle Eastern ancestry. Nor is their majority European ancestry component homogeneous; most of their European ancestors were similar to modern Southern Europeans, while a small minority of Ashkenazi ancestry more closely resembles Northern European populations. Finally, analysis finds almost no genetic substructure in Europe’s Ashkenazim. Lithuanian, Ukrainian, Polish, Romanian and Hungarian Jews all share the same ancestors, as opposed to being descended from distinct, local, regional Jewish populations (as in contrast, the Romaniotes, Greece’s native Jews, for example).
This genetic homogeneity exists because Ashkenazi Jews underwent a massive population bottleneck and demographic expansion more than 500 years ago, consigning them to all share the same fateful core group of common ancestors no matter their more recent country of origin. In population-genetic terms, the generations since 1500 AD are a trivial span, inadequate to have seen the local Jewish populations of Central and Eastern Europe diverge genetically from one another. Though the shtetls, Jewish-dominated towns that characterized this region for many centuries, kept the Jews segregated from local gentiles, they remained interconnected across boundaries of nation and state via robust travel, trade and marriage networks. Eastern European Ashkenazim were fluidly transcending borders for centuries before the advent of the European Union’s Schengen Area.
These genomic insights allowed scientists and historians to sketch at least the rough outline of a narrative about the Ashkenazi emergence. Up until this year, the default story of Ashkenazi ethnogenesis was that about a millennium ago, a group of Jews left Western Europe’s post-Roman territories, Gaul (France), Iberia (Spain) and Italy, emigrating into Germany via the Rhineland. These migrants descended from a mix of various gentile Southern Europeans, Italians and Iberians, plus Jewish migrants from the Middle East. The timing of this population’s emergence is still debated. The key mixing between Jew and gentile may have begun in the centuries after Rome’s fall, or perhaps at the Empire’s height. The paternal ancestry of these Jews derives from the Middle East. Still, they periodically married local women, as evidenced by their many mtDNA lineages that are not distinctively Middle Eastern. Moving eastward across the vast frontier then being opened by the expansion of massive late-medieval and early-modern Poland-Lithuania, these proto-Ashkenazim intermarried with a small number of locals, minimally increasing their European ancestral fraction along a high-birth-rate fueled trajectory to becoming one of Eastern Europe’s largest minorities. It was only then, two centuries ago, that the inexorably inquisitive forces of the Enlightenment brought this singular community of humans to the attention of their fellow Europeans, and the Ashkenazim claimed their prominent role in the intellectual and cultural scene of the continent on which they had resided for well over a millennium.
But details remain to be nailed down in this working draft. I chose my words carefully when I said above that “almost no genetic structure” emerged in the Ashkenazi genomes, because apparently, the small number of Northern Europe Jews who descend entirely from the communities of western Germany are subtly different from the majority of their kinfolk to the east. Josh Lipson, a genetic genealogist focused on Jewish uniparental lineages, emphasized this fact on my podcast last year. Lipson’s argument then was that although we do know the general origin and history of the Ashkenazi Jews, many emerging genetic details add twists or subvert the excessive simplification of the overall story that had been coalescing into a new orthodoxy.
One of Lipson’s contentions was that ancient DNA, specifically medieval DNA, might clear up some of the confusion and hazier sections in the storyline. LIttle over a year later, Lipson has been proven correct. Two papers on Ashkenazi Jewish genetics examining medieval DNA are now out, Genomes from a medieval mass burial show Ashkenazi-associated hereditary diseases pre-date the 12th century, use samples from the English city of Norwich dating to 1190 AD, and Genome-wide data from medieval German Jews show that the Ashkenazi founder event pre-dated the 14th century, in which the team sequenced samples buried in the 1350’s. Lipson and two other specialists in Jewish genetic genealogy are co-authors on the second paper. Lipson was brought on just months after he made that prescient observation on my podcast.
Now that we have medieval Jewish DNA, we can begin to answer some of the questions still pending. Instead of just inferring a phylogenetic tree, we have enough data to concretely probe its internal structure deep into the past. Lipson was spot on about his earlier caution; the subtle differences seen in modern Ashkenazim are but a faint remnant of much greater diversity in the people’s medieval past. The bottleneck whose echo is unmistakable in modern Ashkenazi Jewish genomes was actually at minimum two independent bottlenecks, and at least one of them was a far tighter bottleneck than the other. Up until now, statistical methods have been faithfully detecting the composite demographic history that we’d assumed was singular (that being the most parsimonious explanation, given our limited data). The likelihood of very strong bottlenecks is the most efficient explanation for modern Ashkenazim’s high load of recessive diseases, which we can now see predate the medieval period. The medieval Ashkenazim sampled were a combination of Southern and Northern Europeans, as well as ancient Middle Easterners, as we already believed. But now, we know the modern Ashkenazim are a fusion of distinct and disparate Jewish communities that established separate footholds in Northern Europe in the centuries around 1000 AD, perhaps dozens of small diasporas, many still awaiting discovery.