RKUL Hit List 2023: Ashkenazi Jewish genetics, a match made in the Mediterranean
Revisiting favorite Substack pieces, with research-based revisions
The main project of this Substack is writing the first drafts of newly genetics-informed human population histories. Over the past two decades, genomics has been deluged with manifold riches on an absolutely staggering scale. Computational power, DNA extraction scale (especially from ancient samples) and widespread affordable consumer testing have all gone from intangible pipe dreams to tearing through phases of literally exponential growth. Our powers have risen multiple orders of magnitude in the last 20 years. I have been an excitable front-row commentator on the genomic revolution from the perch of my original blog for two decades now, but this new project of systematically covering what we thought we knew of human history and population movements versus what genomics can confirm or rule out is only a little over three years old for me.
The first full year I published this newsletter, 2021, I began diving into reflections on some of my top favorite historic and prehistoric human populations. People were paying me to write about my favorite stories; I went straight for my own canon of greatest hits first. But I had no idea whether this mailing list would keep growing and most of you weren’t here yet. These are some of the richest human stories I know and only a select few insiders were reading those first drafts. An exciting thing about writing up papers in human population genetics as they’re released… is that as spot-on as they might prove, they’re rarely the last word. Exciting or plot-twisting addenda and further details are always likely to appear around the bend.
So, as the third full calendar year of this Substack draws to a close, I’m revisiting three of my favorite past posts, updating them to reflect the assorted subsequent findings I’d include if I were first writing those drafts today. When this piece was published in September of 2021 we knew a lot about Jewish genetics. But now, we know a lot more, and I added many substantive changes to the piece (bolded). Those research-driven updates led me to rework the text generally, so even if you were one of the 1,430 paying subscribers who originally received it, today's piece is a refreshed read. Now, it goes out to some 39,000 total free and paid subscribers.
Again, for all who have already read it, today’s updates appear in bold so you can easily scroll between them.
Sometime after the year 1000 AD, a group of Jews began migrating eastward across Europe, into the principalities of Germany and the kingdom of Poland, attracted by the combination of religious tolerance and economic opportunity. These territories were beyond the frontiers of the Roman Empire; they were lands that Jews had not traditionally occupied. By the time these pioneers arrived in the small towns of Germany and the hamlets of Poland, Jewish communities had already been established in Persia and Egypt for 1,500 years. Within a matter of generations, the Jews migrating out of France into Germany encountered another community of believers who had emerged out of the east. These latter were Slavic-speaking Jews whose ultimate provenance remains even more opaque than the westerners’. Though the cultural touchstones and self-identity of the later Ashkenazim derive primarily from the western Jews whose forebears sojourned in France, producing incredibly influential rabbis like Rashi of Troyes, the eastern migrants’ enduring mark on the European Jewish gene pool Jews has become unmistakable with the advent of modern genetics.
For the next eight centuries, this emergent Jewish population, an amalgam of western and eastern proto-Ashkenazim, waxed in numbers thanks to their critical economic position in the developing lands of Eastern Europe. But unlike the Hebrews of antiquity, they became culturally invisible to their gentile neighbors, quietly navigating a closed social universe organized around adherence to their own laws and focused on their own texts. Their ultimate origins were a mystery to the gentiles around them, and indeed soon lost to time even among themselves. Were they the descendants of the ancient Hebrews, converts to the religion, or a mix of both? The very range of possible origins was soon scarcely even grasped by Eastern European Jewry as any faithful memory of their past faded and their written culture focused purely on matters of religion.
This curiosity is worth pausing over. Ashkenazi Jewish luminaries have recorded such outsized contributions to every aspect of human output in culture and knowledge over the past two centuries, it’s natural to assume that the familiar intellectual restlessness and insatiable curiosity of the community’s standouts have deep cultural roots. But if the textual record is anything to go by, nothing could be further from the truth. Hunting for attested evidence of Ashkenazi Jewish passage from antiquity to the 19th century is like trying to catch a glimpse of a secretive nocturnal creature. Not only did Jews receive unhelpfully scant coverage from gentile chroniclers, the community itself also appears to have trained its considerable literacy and intellectual power solely on matters Talmudic, to the complete exclusion of any historical records of the various communities.
And yet, it was these people who flourished in Eastern Europe’s unknown wild lands who would go on to beget 80% of the modern Jewish diaspora, and join the mainstream of Western civilization by the 19th century. These are the more than ten million Ashkenazim, whose members, since their reintegration into the stream of post-Enlightenment Western civilization, have left indelible marks on world history and culture so far out of proportion to their numbers, from Albert Einstein to Karl Marx.
In the Bible, Ashkenaz is one of the descendants of Noah, and Jewish scholars speculatively associated his scions with various points north, initially Scythia, but eventually Germany. So the Ashkenazim were the Jews of Ashkenaz, of Germany and parts east, and their native language, Yiddish, was a dialect of German. The Jews of Spain and Portugal, the Sephardim, loomed large during the medieval period between 1000 and 1500 AD, producing the great rabbi Moses Maimonides and polymath Judah Halevi, and persisting in prominence into early modernity, with philosophers like Baruch de Spinoza. It was only in the 19th and 20th centuries that the history of Jewish prominence and cultural achievement became so disproportionately the story of the Ashkenazim, who were 92% of the world’s Jews in 1930, and have shared in fully 20% of all Nobel Prizes awarded.
And yet, though the Jews are a people whose history is extensively documented, from the Bible to Josephus’ Roman-era The Jewish Wars, the origins of the Ashkenazim remain enigmatic if we limit ourselves to textual sources. In 1096 AD, Christian crusaders infamously massacred Jews in the German Rhineland as warm-up for the slaughter they would inflict upon Middle Eastern peoples, and the German-speaking lands saw widespread pogroms in the mid-14th century, at the height of the Black Death. But in comparison to their ubiquity in the 19th century, the Ashkenazim are mentioned only glancingly in the histories of this earlier period. They rose to be notable only upon their demographic ascendence in the massive dominion of Poland-Lithuania, as the Middle Ages gave way to the Renaissance five centuries ago. Not to mention in their later cultural explosion in the modern world.
Given Ashkenazi Jews’ newfound prominence and nebulous origins, late 19th-century European intellectuals began exploring the so-called “Khazar hypothesis” for the origin of the Ashkenazim: the curious idea that Eastern European Jews descended from an ancient Turkic steppe confederacy destroyed by Kievan Rus 1,050 years ago. The Khazars are notable because much of their elite reputedly converted to Judaism while other groups were adopting Christianity or Islam. The Khazar hypothesis’ argument ran that the Ashkenazim descended from the scattering of Khazar Jews westward into Europe. It is only with genetics in the 21st century that we have been able to really test this theory,. ultimately finding it mostly wanting. The Ashkenazim are primarily the synthesis of ancient Levantine Jews and various Mediterranean European populations with whom the former mixed. Ashkenazi origins date to Rome’s fall, not Khazaria’s.
But now as we sift through ever more genomic data, both modern whole genomes and groundbreaking medieval DNA from England and Germany, we begin to discern a faint glimmer kindling back into flame that might indeed reflect a true Khazar connection, tenuous and distant though it would be: the Ashkenazim descend in part from those hard-to-place eastern Jews who settled in the early medieval state of Great Moravia, centered on the modern territories of the Czech Republic and Slovakia. It was these Slavic-speaking Jews who contributed Northern European ancestry to the Ashkenazi mix. And so they also now emerge as the strongest candidate vector for what is today a very small fraction of East Asian ancestry occasionally found in modern Ashkenazi Jewish genomes, a heritage that may in fact trace back to the Khazar state that once dominated the steppe to Moravia’s east.
In the intersection
To understand when and where the Ashkenazim come from, it is important to understand what they were before they became the distinctive people we know from history and fiction. The ancient King David was a simple shepherd, while the Babylonian Talmud outlines how farmers must maintain adherence to the laws of the Torah despite the agricultural season’s cycles. At the Roman Empire’s height 2,000 years ago, the Jews were both pedestrian and unique. Pedestrian in that they were a nation of farmers and shepherds, as the Bible extensively documents. Yes, large communities of urban Jews flourished in Alexandria, Rome and other large cities, but on balance, the Jews were not particularly urban. Like the Jews, Greeks outside of their homeland tended to be urban as well, but the average Greek was still a farmer or a shepherd, as were the vast majority of humans in the ancient world (not to mention, incidentally in the world as a whole until the 20th century). The ancient Jews were tillers of the soil and drivers of flocks, like all their contemporaries.
Where the Jews were unique was their strict adherence to a set of laws handed down to them by a god whom they held to be the one true god above all others. The Jews were zealous in their religious particularity, a reality which led to a war of liberation against the Greeks in 167 BC, where they rebelled against the imposition of pagan syncretism by raising an altar to Zeus in the Jewish Temple. Today, we know of this war mostly through its annual commemoration at Hanukkah. Once Rome rose and conquered the Eastern Mediterranean, the Jews rebelled twice against the Roman imperium, once in the first century AD and once in the second. Again, their rebellion was sparked when they chafed against the rule of the religiously tolerant but unabashedly polytheistic Romans. The first rebellion was triggered by riots that erupted in 66 AD when pagan Greeks provocatively sacrificed birds in front of a synagogue, an act of sacrilege in Jewish eyes intended to inflame tensions in Jerusalem. The first rebellion ended a period when Jews were prominent in Roman elite circles, particularly due to the Judaean client king Herod Agrippa’s friendship with the Roman Emperor Caligula. From then on, Jews were tolerated, but seen to be different: a people apart.
The Jewish farmers and warriors who characterized the nation in the first centuries of the Common Era would eventually fade from living history, recalled only as legends in scripture and oral tradition. By the time European Jews became more than marginal curiosities in early modernity, pure subsistence agriculture and a martial ethos had become wholly alien to the almost purely urban and small-town Ashkenazim’s mode of existence. The Zionist movement made explicit efforts to synthetically re-infuse the ethos in Palestine’s new settlers. Zionism emerged from socialism and 19th-century nationalism and imagined a robust patriotic citizenry working the land on collective farms, the kibbutzim, standing ready to rise as a nation and take up arms against enemies near and far. Set against this future ideal was the contemporary bourgeois life that was aspiration and reality for many of the European Ashkenazim, who had already transitioned to comfortable material security in the wake of the piecemeal Jewish emancipation that swept the continent over the course of the 19th century.
But these assimilated Ashkenazim still came from an earlier regime, where Jews were set apart from the nations among whom they dwelt. Whereas their biblical ancestors had been farmers, pastoralists and warriors, the Ashkenazim known from later medieval and early modern history occupy professions avoided by Christians. The more modest members of the community were peddlers and artisans serving rural villages, while the Jewish elite were money-lenders and tax-farmers, intermediaries between the aristocracy that ruled much of Europe and the peasants whom they exploited. The enmity toward the Ashkenazim pervasive across much of Europe in the early modern period derived from this experience, as the dirty work of wringing taxes from immiserated farmers fell to the Jewish subordinates of rural nobility.
Meanwhile, in the domain of high finance, Jewish families like the Warburgs and Rothschilds would make their mark as lenders to kings, weathering all the risks and exulting in the windfalls that ensued.
These opportunities presented themselves because the landscape the early Eastern European Jews entered was itself only emerging out of Iron-Age subsistence. This world urgently needed their skills and services as it began to ascend the scale of economic and social complexity. The Jews of Lithuania were first granted an official charter in 1388 AD by a duke Vytautas to settle and trade in his territories, one year after Lithuania’s conversion to Christianity. It is almost certain that Jews were already present when the territory was wholly pagan, likely for decades, judging from the fact that the capital Vilnius is definitively known to have hosted a large Muslim merchant community as far back as the 13th century.
In contrast to the primitive state of affairs in Northern Europe, in the Roman Empire, Jews were but one among many peoples. They were mostly peasants, albeit peasants with an identity rooted in traditions and text, and a strong self-conception as a nation. In that way, they were similar to the Greeks, with their Iliad, and later the Romans, with their Aeneid. When the Jews moved east of the Rhine 1,000 years ago, or up from the Roman Empire beyond the Pannonian frontier and into the territory of Great Moravia, they were venturing into lands that had never known the hand of Rome, where the touch of Christianity was light or nonexistent, and illiterate barbaric warlords styled themselves dukes and kings. The Ashkenazim and their forebears occupied a slot in the social and political system of societies that were fundamentally different from the post-Roman world from which they’d come, with its deep traditions of literacy and cultural continuity preserved in a Christian Church with roots back to the Roman Empire. In the process, the Ashkenazim blazed new trails and drew the template for what it would come to mean to be a Jew in the second half of the Common Era. As Northern Europe rose, so did they.
A people beyond geography
In most of Europe, genetic variation is neatly predicted by geography. Genetically, the French are more similar to Germans than they are to Russians. Russians are more similar to Poles than they are to Swedes. And so forth. But there are exceptions. The Ashkenazim are scattered across numerous nation-states. In the early 20th century, almost every nation north of Italy, Greece and Spain, and south of Scandinavia, had its own large population of Ashkenazi Jews. Though the Holocaust and World War II destroyed this vibrant diaspora, millions of Americans, Israelis, French and Britons descend from Jewish Lithuanians, Poles, Hungarians, Romanians, Russians and Germans. And yet genetically, these Jews of various nationalities are quite similar. A Lithuanian Jew is genetically far closer to a Bavarian Jew than to his gentile Lithuanian neighbor (the main exception to this truism is the very small community of indigenous Rhenish Jews in western Germany, who seem to have less Northern European ancestry than the vast majority of Ashkenazim with origins in Poland-Lithuania, likely reflecting this distinct and isolated community’s minimal contact with Moravian Jews in the medieval period).
The similarity within Ashkenazi populations is not entirely surprising, because by the Renaissance their peregrinations were documented in chronicles, and their shared origins evident in their common Yiddish language and culture. Most of Germany was empty of Jews in the 18th century, due to early expulsions in the Middle Ages and the chaos of the Wars of Religion in the 17th century. So where did they come from? The Rhenish Jews of the Rhineland’s mercantile cities had a deep continuity in the region despite famously having suffered pogroms in the Middle Ages. But all the available evidence points to the main demographic pulse coming from the east. The Jews of the Haskalah, the 19th-century Jewish Enlightenment, were predominantly German-speaking, but the new “Germans of the Mosaic faith” were mostly recent descendants of Eastern European migrants. Settling down in the cities of Germany, these burghers quickly forgot their ancestors’ precise origins and produced intellectuals of the caliber of philosopher Moses Mendelssohn, whose descendents fully assimilated to gentile culture. When subsequent waves of Jews from the east arrived, the settled German Jews expressed horror at their habits and customs, contemptuously referring to them as “Ostjuden.” But by and large they were in fact one people, with one genetic origin.
If you are Ashkenazi Jewish and search for relatives on 23andMe, the widespread relatedness is clear. Whether your ancestors are German, Hungarian or Russian, you find distant “relatives” galore, and only minimal correlation with geography. This is because just as the histories and culture suggest, genetics reflects the reality that the Ashkenazim all derive from a small number of common ancestors. They clearly underwent a population bottleneck, where the numerous lineages of Jews in the past were winnowed down to only a few, who eventually resurged to give rise to the millions of Ashkenazim. Whereas the ancestors of French gentile peasants may have been residents in their home province for 1,000 years, the ancestors of today’s Ashkenazim moved from principality to principality, fleeing persecution, chasing opportunities. In the process, they intermarried exclusively with only a small number of fellow co-religionists.
One of the likely consequences of this small founder population is Jewish genetic diseases. When people intermarry with only those they are related to, even if quite distantly, genetic variants associated with disease tend to eventually escape the grip of natural selection and shuck off the mask of a recessive state. Tay-Sachs, Torsion Dystonia and Gaucher’s disease are just three of the heritable ailments associated with Ashkenazi Jews, though not exclusive to them. Though some have proposed natural selection as driving these diseases within the Ashkenazim, most researchers accept the contention that a small effective founding population is the culprit.
The Age of DNA
Though the reality of Jewish genetic distinctiveness was clear from the existence of particular diseases within the community, a deeper exploration of Ashkenazi genetics had to await the 21st century. Despite extensive pseudo-scientific attempts to classify the Jews in the early 20th century, the tools of science were inadequate to the task of human taxonomy at such a fine-grained level until recently. Now with the whole human genome as the data bank from which to draw information, the field of Jewish genetics has yielded a wealth of insights related to ancestry and heritage.
First, around the year 2000, geneticists looked at the direct paternal lineage on the Y chromosome for two particular Jewish groups, the Cohens and Levites. These are two priestly lineages, traditionally defined through common descent from a shared ancestor, Moses’ brother Aaron, and Jacob’s son Levi, respectively. The Rabbinical Judaism that coalesced during the Roman Empire focuses on direct maternal lineage to define membership in the Jewish nation, but the paternal inheritance custom of the two priestly lineages reflects a more ancient tradition going back to the Bible.