Scholars of all kinds have long debated one seemingly simple question: What is "Jewishness?" Is it defined by genetics, culture, or religion? Recent findings have revealed genetic ties that suggest a biological basis for Jewishness, but this research didn’t include data from North African, Ethiopian, or other Jewish communities. Now a new study fills in the genetic map—and paints a more complex picture of what it means to be Jewish.
Modern Jews, who number more than 13 million worldwide, are traditionally divided into various groups. They include Middle Eastern Jews, who live in Iraq, Iran, and other places in the Levant; Sephardic Jews from Spain and Portugal; Ashkenazi Jews from Europe, who comprise 90% of American Jews; North African Jews from Morocco, Algeria, and other countries north of the Sahara; Ethiopian Jews; and many other communities scattered across the globe. In the Bible, the roots of Jewishness reach back 4000 years to Abraham and his descendants. But historians have suggested the story of Jewishness is more complicated, and may not include a single ancestor. Some have even argued that most modern Jews are descended from converts to Judaism and don’t share genetic ties at all.
Recent studies have turned to DNA for answers. In 2010, human geneticist Harry Ostrer of the Albert Einstein College of Medicine in New York City and colleagues found that three of the major Jewish groups—the Middle Eastern, Sephardic, and Ashkenazi Jews—share a genetic connection going back more than 2000 years, and are more closely related to each other than to nearby non-Jewish groups. Genetic ties within each of the groups were even closer, about the equivalent of fourth or fifth cousins. But that study didn't include North African Jews, who represent the world's second largest Jewish population, or any groups whose claim to Jewishness has been controversial, such as Ethiopian Jews.
So Ostrer and his colleagues gathered new DNA samples from Jews living everywhere from Morocco to Yemen. Using three distinct strategies for identifying genetic similarities, including a method called identity by descent (IBD) that can determine how closely related two individuals are, the team compared these DNA samples to each other, to the samples from their 2010 study, and to samples from non-Jews. Most of the sampled groups shared genetic features, indicating a common heritage dating back to before Roman times, the team reports today in the Proceedings of the National Academy of Sciences. North African Jews—and Moroccan/Algerian Jews in particular—showed a close genetic connection to Ashkenazi and Sephardic Jews, and little evidence of interbreeding with contemporary non-Jewish populations in North Africa. Georgian Jews shared genetic features with Middle Eastern Jews, instead. Yemenite Jews were distantly related to Middle Eastern Jews, while Ethiopian Jews formed their own cluster and shared little IDB with other Jewish populations. Each group showed little interbreeding with local non-Jewish groups. Moroccan/Algerian Jews, for example, were about as close genetically as third or fourth cousins; Jews from the Tunisian Island of Djerba were as close as first cousins once removed.
"I didn’t know what to expect," Ostrer says. "I've been surprised to learn there's such a shared biological basis for Jewishness." The team's results suggest that while most Jewish groups are genetically related, some are not and instead arose from converts to Judaism. But regardless of their origins, Jewish groups remained genetically isolated once formed.
The results complement historical accounts of multiple Jewish migrations and expulsions. The genetic ties between North African Jews and Ashkenazi and Sephardic Jews may reflect the expulsion of European Jews from Spain and Portugal during the Spanish Inquisition in the late 1400s, and their limited breeding with local North African populations in the centuries that followed. Distinct populations, such as Ethiopian Jews, likely arose from Jewish founders who converted the local population by proselytizing but did not intermarry. "This is certainly the most extensive genomic study of Jewish populations to date," says geneticist Sarah Tishkoff of the University of Pennsylvania, who was not involved in the work. "And it shows there's both a genetic and a cultural component to being Jewish."
Identifying the genetic component of Jewishness—though controversial because the Holocaust was predicated on the idea that Jewishness was a genetic trait that could be eliminated from the German population—could have medical as well as historical value, Tishkoff adds, because many Jewish populations have high incidences of genetic disease. Knowing more about the groups' biological makeup could enable doctors to provide more informed genetic counseling to Jewish couples, or better personalize courses of treatment. Tishkoff notes that the little-studied Jewish populations of India, sub-Saharan Africa, China, and Burma weren’t examined in the latest analysis. Ostrer says his team plans to include their DNA in a future study to complete what he calls "the tapestry of Jewishness."