Jewish DNA Speaks
Reprinted from Jewish Ideas Daily.
Are Jews a "nation" or a "people"? The Hebrew term am means both. Both terms, moreover, have been subjected to disapprobation in our time—although not nearly to the extent of "race," a term that Jews themselves stopped using nearly a century ago. How, then, are we to think about the mounting genetic evidence that points to Jewish biological continuity over time?
The field of genetics has been offering up sensational new observations about the historical record of Jewish origins, exile, and migrations. On the men's side of the aisle, one of the most dramatic discoveries is that both Ashkenazi and non-Ashkenazi kohanim—traditionally, descendants of the biblical high priest Aaron—share an extended haplotype or DNA sequence variation that does indeed distinguish them from other Jews (as well as non-Jews). The divergence is estimated to have taken place about 3,200 years ago (plus or minus a thousand years): that is, well before the dispersion of the Jewish people into communities around the Middle East and Europe. Although it remains difficult to say whether the lineage traces to Aaron himself, scientific research does support the self-identification of many or most kohanim today.
In the women's section, the results are equally profound. Analysis of mitochondrial DNA, which is passed down only by mothers, indicates that four lineages—that is, four specific women in the Middle Ages—were the originators of 40 percent of the entire Ashkenazi population. Somewhere before the 12th century, these four founders, whose own genetic ancestors hailed from the Near East, appeared in Europe, probably in the Rhine Valley, to become the matriarchs of much of the Ashkenazi world. There may then have followed a long period of "bottleneck," without significant population growth, in which mutations may have manifested themselves in the form of genetic disorders. Such characteristically Ashkenazi diseases as Tay-Sachs may have been one byproduct of such group cohesion and slow growth.
Similar founder events appear in other regions. Jews of Mumbai can trace their descent back to five women, while Jewish communities in Dagestan and Georgia trace three-fifths of their genetic variations to one woman in each region. Forty-three percent of Iraqi Jews descend from five women; among Iranian Jews, 41 percent descend from another six.