After a genome-wide analysis last week reported that Jews worldwide have significant genetic commonalities, a second study published today comes to a similar conclusion. But will the new evidence put to rest a debate that has lasted more than a century?

Tradition has it that Jews around the world are one people with shared religious and cultural practices. Several studies from the last decade that have looked closely at genetics have pointed to a common history based on shared genetics between different Jewish populations. But many historians doubt that genes could be shared, noting that there was so much mixing between Jews and neighboring populations – non-Jews converting to Judaism in large numbers around or before the time of Christ, or Jews being sold as slaves and sent off to far-off lands, then freed there – that they should have come to genetically blend into the geographical population where they ended up.

Catherine Hezser, who studies ancient Judaism at the University of London’s School of Oriental and African Studies and wasn’t involved in the research, notes that for this reason, she does not “believe in any of the genetic theories, which are usually propagated for non-scientific, ideological reasons”. But the researchers say their fine-grained analysis of 14 Jewish populations, published in Nature today, could provide some answers.

The groups examined in the study included not just major communities such as Ashkenazi Jews of Eastern Europe and Sephardim from Bulgaria and Turkey, but also several that are much smaller and, like the Bukharan Jews of Central Asia and Jews from India, Ethiopia and Yemen, are often referred to as the “lost tribes” of Israel. The study mainly examined single-letter variations – called single nucleotide polymorphisms (SNPs) – in individuals within each group, compared to individuals in surrounding areas.

Much like the analysis published last week, the study showed that all of the Jewish communities share some common genetic features, and for the most part, the Jewish groups are more similar to each other than to the non-Jews in the same regions.

“These two studies are the first pair of genome-wide studies of SNP variations in collections of multiple Jewish populations,” says Noah Rosenberg, a population geneticist at the University of Michigan who was not involved in either study. The strength of the Nature paper, he says, is that it compared the Jewish groups to an unprecedentedly broad array of non-Jewish groups, making the comparison – and the genetic ties identified – especially robust.

Because of their large panel of populations, the researchers were able to dive more deeply than ever before into fine scale relationships between different populations. The closest genetic clustering, both among Jewish and non-Jewish groups, is seen in the eastern Mediterranean area known as the Levant, including Israel, Lebanon, Syria, and surrounding regions, the study finds. One interpretation, then, says Tudor Parfitt, also of the University of London’s School of Oriental and African Studies and an author on the Nature study, may be that the most active mixing between Jews and non-Jews occurred there. That is, if those two groups were already genetically very close, such admixture wouldn’t have changed much.

Meanwhile, Ethiopian Jews and those from Mumbai, India, seem to be much closer genetically to their host populations than are other groups examined. These groups, says Parfitt, probably did largely come to Judaism more by conversion than by migrating from larger Jewish communities; in that sense, even the limited genetic ties the researchers identified were surprising, he notes. At the same time, stresses Richard Villems, a molecular anthropologist at the University of Tartu in Estonia and one of that study’s lead authors, “if the genes seem to tell a story that some Jewish communities are further away from the main communities, that doesn’t make them any less Jewish”.

Karl Skorecki at Technion University in Haifa, a nephrologist, occasional population geneticist, and a lead author on the study, notes that the two SNP studies together lay a foundation for studying disease-related genes that might be specific to Jewish people. Some questions to ask, he says, might be how do certain alleles become specific to certain populations, and how do periods of active mixing with other groups influence genome-wide structure. “One can start to do very rigorous analyses and extract them to other populations on the planet,” he adds.

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