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Tracing the Roots of Jewishness
3 June 2010 12:46 pm
Who are the Jews? For more than a century, historians and linguists have debated whether the Jewish people are a racial group, a cultural and religious entity, or something else. More recently, scientists have been weighing in on the question with genetic data. The latest such study, published today in the American Journal of Human Genetics, shows a genetic connection among all Jews, despite widespread migrations and intermarriage with non-Jews. It also apparently refutes repeated claims that most Ashkenazi Jews are descended from Central Europeans who converted to Judaism 1000 years ago.
Historians divide the world's 13 million living Jews into three groups: Middle Eastern, or Oriental, Jews; Sephardic Jews from Spain and Portugal; and Ashkenazi Jews from Europe. Although the Bible traces Jewish roots back to the time of Abraham some 4000 years ago, most historians have concluded that the actual Jewish identity dates to only a little over 2000 years ago.
The origins of today's Jews have been less clear, especially those of the Ashkenazis, who make up 90% of American Jews and nearly 50% of Israeli Jews. Ashkenazi Jews settled in Germany in the 9th century C.E. and developed their own language, Yiddish. Some writers, notably Arthur Koestler in his 1976 book The Thirteenth Tribe, have argued that the Ashkenazis stem from a Turkic tribe in Central Asia called the Khazars, who converted to Judaism in the 8th century. And historian Shlomo Sand of Tel Aviv University in Israel argues in his book The Invention of the Jewish People, translated into English last year, that most modern Jews do not descend from the ancient Land of Israel but from groups that took on Jewish identities long afterward.
Such notions, however, clash with several recent studies suggesting that Jewishness, including the Ashkenazi version, has deep genetic roots. In what its authors claim is the most comprehensive study thus far, a team led by geneticist Harry Ostrer of the New York University School of Medicine concludes today that all three Jewish groups—Middle Eastern, Sephardic, and Ashkenazi—share genomewide genetic markers that distinguish them from other worldwide populations.
Ostrer and his colleagues analyzed nuclear DNA from blood samples taken from a total of 237 Ashkenazi and Middle Eastern Jews in New York City and Sephardic Jews in Seattle, Washington; Greece; Italy; and Israel. They compared these with DNA from about 2800 presumably non-Jewish individuals from around the world. The team used several analytical approaches to calculate how genetically similar the Jewish groups were to each other and to the non-Jewish groups, including a method called identity by descent (IBD), which is often used to determine how closely two individuals are related.
Individuals within each Jewish group had high levels of IBD, roughly equivalent to that of fourth or fifth cousins. Although each of the three Jewish groups showed genetic admixture (interbreeding) with nearby non-Jews, they shared many genetic features, suggesting common roots that the team estimated went back more than 2000 years. Ashkenazi Jews, whose genetic profiles indicated between 30% to 60% admixture with Europeans, nevertheless clustered more closely with Middle Eastern and Sephardic Jews, a finding the researchers say is inconsistent with the Khazar hypothesis. "I would hope that these observations would put the idea that Jewishness is just a cultural construct to rest," Ostrer says.
Other researchers praise the work. It's the largest to date on this question, and using the IBD method to tackle it is "innovative," says geneticist Noah Rosenberg of the University of Michigan, Ann Arbor. Sarah Tishkoff, a geneticist at the University of Pennsylvania, says that "this is clearly showing a genetic common ancestry of all Jewish populations." Nevertheless, says Rosenberg, although the study "does not appear to support" the Khazar hypothesis, it doesn't entirely eliminate it either.
The study does not address the status of groups whose claim to Jewishness has been controversial, such as Ethiopian Jews, the Lemba from southern Africa, and several groups from India and China. But given the findings of a common genetic origin plus a complex history of admixture, geneticist David Goldstein of Duke University in Durham, North Carolina, says that neither of the "extreme models"—those that see Jewishness as entirely cultural or entirely genetic—"are correct." Rather, Goldstein says, "Jewish genetic history is a complicated mixture of both genetic continuity from an ancestral population and extensive admixture."