Tuesday, October 8, 2013
Are Ashkenazi Jews descended from Italian women?
Over the years geneticists have identified links between the world’s Jewish communities that point to a common ancestry as well as a common religion. Still, the origin of one of the most important Jewish populations, the Ashkenazim of Central and Eastern Europe, has remained a mystery. A new genetic analysis claims to have filled in another piece of the origins puzzle, pointing to European women as the principal female founders, and to the Jewish community of the early Roman empire as the possible source of the Ashkenazi ancestors. The finding establishes that the women who founded the Ashkenazi Jewish community of Europe were not from the Near East, as previously supposed, and reinforces the idea that many Jewish communities outside Israel were founded by single men who married and converted local women. The study is based on a genetic analysis of maternal lineages. A team led by Martin B. Richards of the University of Huddersfield in England took a fresh look at Ashkenazi lineages by decoding the entire mitochondrial genomes of people from Europe and the Near East. Earlier DNA studies showed that Jewish communities around the world had been founded by men whose Y chromosomes bore DNA patterns typically found in the Near East. But there was a surprise when geneticists turned to examine the women founders by analyzing mitochondrial DNA, a genetic element that is separate from the main human genome and inherited just through the female line. Unlike the Y chromosomes, the mitochondrial DNA showed no common pattern. In several of the smaller Jewish communities it clearly resembled that of the surrounding population, suggesting a migration pattern in which the men had arrived single, perhaps as traders, and taken local wives who then converted to Judaism. But it wasn’t clear whether or not this was true of the Ashkenazim. Mitochondrial DNA tends to change quite rapidly, or to drift, as geneticists say, and the Ashkenazi DNA has drifted so far it was hard to pinpoint its origin. This uncertainty seemed to be resolved by a survey published in 2006. Its authors reported that the four most common mitochondrial DNA lineages among Ashkenazis came from the Near East, implying that just four Jewish women were the ancestresses of nearly half of today’s Ashkenazim. Under this scenario, it seemed more likely that the Ashkenazim were the result of a migration of whole communities of men and women together. But decoding DNA was still quite expensive at that time and the authors of the 2006 survey analyzed only a short length of the mitochondrial DNA, containing just 1,000 or so of its 16,600 DNA units, in all their subjects. The four mitochondrial lineages common among Ashkenazis are now very rare elsewhere in the Near East and Europe, making it hard to identify with certainty the lineages from which they originated. With the entire mitochondrial genome in hand, Dr. Richards could draw up family trees with a much finer resolution than before. His trees show that the four major Ashkenazi lineages in fact form clusters within descent lines that were established in Europe some 10,000 to 20,000 years ago. The same is true of most of the minor lineages. “Thus the great majority of Ashkenazi maternal lineages were not brought from the Levant, as commonly supposed,” Dr. Richards and colleagues conclude in their paper. Overall, at least 80% of Ashkenazi maternal ancestry comes from women indigenous to Europe, and 8% from the Near East, with the rest uncertain, the researchers estimate. Dr. Richards estimates that the four major lineages became incorporated into the Ashkenazi community at least 2,000 years ago. A large Jewish community flourished in Rome at this time and included many converts. This community could have been the source of both the Ashkenazim of Europe and the Sephardim of Spain and Portugal, given that the two groups have considerable genetic commonality, Dr. Richards said. Doron M. Behar, of the Gene by Gene company in Houston and a co-author of the 2006 survey, said that he disagreed with Dr. Richards’ conclusions but declined to explain his reasons, saying that they had to appear first in a scientific journal. David B. Goldstein, a geneticist at Duke University who first detected the similarity between the founding mothers of Jewish communities and their host populations, said that the new analysis was well done but that the estimate of 80% European origin for the Ashkenazi maternal lineages was not statistically justified, given that mitochondrial DNA lineages rise and fall in a random way. A recent analysis based on the whole genomes, not just mitochondrial DNA, of Jewish communities around the world noted that almost all overlap with non-Jewish populations of the Levant, “consistent with an ancestral Levantine contribution to much of contemporary Jewry.” Dr. Richards said that the finding was compatible with his own, given that the Levantine contribution was not that great. Another recent study, also based on whole genomes, found that a mixture of European ancestries ranged from 30% to 60% among Ashkenazi and Sephardi populations, with Northern Italians showing the greatest proximity to Jews of any Europeans. The authors of this study, led by Gil Atzmon of the Albert Einstein College of Medicine, noted that there had been mass conversions to Judaism in the early Roman empire, resulting in some 6 million citizens, or 10% of the population, practicing Judaism. Dr. Richards sees this as a possible time and place at which the four European lineages could have entered the Jewish community, becoming very numerous much later as the Ashkenazi population in northern Europe expanded from around 25,000 in 1300 A.D., to more than 8.5 million at the beginning of the 20th century.