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Semitic Genetics

With a new technique based on the male or Y chromosome, biologists have traced the diaspora of Jewish populations from the dispersals that began in 586 B.C. to the modern communities of Europe and the Middle East.

The analysis provides genetic witness that these communities have, to a remarkable extent, retained their biological identity separate from their host populations, evidence of relatively little intermarriage or conversion into Judaism over the centuries.

Jews, Palestinians, and Syrians share a genetic link.

Another finding, paradoxical but unsurprising, is that by the yardstick of the Y chromosome, the world's Jewish communities closely resemble not only each other but also Palestinians, Syrians and Lebanese, suggesting that all are descended from a common ancestral population that inhabited the Middle East some four thousand years ago.

Dr. Lawrence H. Schiffman, chairman of the department of Hebrew and Judaic Studies at New York University, said the study fit with historical evidence that Jews originated in the Near East and with biblical evidence suggesting that there were a variety of families and types in the original population. He said the finding would cause "a lot of discussion of the relationship of scientific evidence to the manner in which we evaluate long-held academic and personal religious positions," like the question of who is a Jew.

The study, reported in today's Proceedings of the National Academy of Sciences, was conducted by Dr. Michael F. Hammer of the University of Arizona with colleagues in the United States, Italy, Israel, England and South Africa. The results accord with Jewish history and tradition and refute theories like those holding that Jewish communities consist mostly of converts from other faiths, or that they are descended from the Khazars, a medieval Turkish tribe that adopted Judaism.

The analysis by Dr. Hammer and colleagues is based on the Y chromosome, which is passed unchanged from father to son. Early in human evolution, all but one of the Y chromosomes were lost as their owners had no children or only daughters, so that all Y chromosomes today are descended from that of a single genetic Adam who is estimated to have lived about 140,000 years ago.

In principle, all men should therefore carry the identical sequence of DNA letters on their Y chromosomes, but in fact occasional misspellings have occurred, and because each misspelling is then repeated in subsequent generations, the branching lineages of errors form a family tree rooted in the original Adam.

These variant spellings are in DNA that is not involved in the genes and therefore has no effect on the body. But the type and abundance of the lineages in each population serve as genetic signature by which to compare different populations.

Based on these variations, Dr. Hammer identified 19 variations in the Y chromosome family tree.

The ancestral Middle East population from which both Arabs and Jews are descended was a mixture of men from eight of these lineages.

Among major contributors to the ancestral Arab-Jewish population were men who carried what Dr. Hammer calls the "Med" lineage. This Y chromosome is found all round the Mediterranean and in Europe and may have been spread by the Neolithic inventors of agriculture or perhaps by the voyages of sea-going people like the Phoenicians.

Another lineage common in the ancestral Arab-Jewish gene pool is found among today's Ethiopians and may have reached the Middle East by men who traveled down the Nile. But present-day Ethiopian Jews lack some of the other lineages found in Jewish communities, and overall are more like non-Jewish Ethiopians than other Jewish populations, at least in terms of their Y chromosome lineage pattern.

The ancestral pattern of lineages is recognizable in today's Arab and Jewish populations, but is distinct from that of European populations and both groups differ widely from sub-Saharan Africans.

Each Arab and Jewish community has its own flavor of the ancestral pattern, reflecting their different genetic histories. Roman Jews have a pattern quite similar to that of Ashkenazis, the Jewish community of Eastern Europe. Dr. Hammer said the finding accorded with the hypothesis that Roman Jews were the ancestors of the Ashkenazis.

Despite the Ashkenazi Jews' long residence in Europe, their Y signature has remained distinct from that of non-Jewish Europeans.

On the assumption that there have been 80 generations since the founding of the Ashkenazi population, Dr. Hammer and colleagues calculate that the rate of genetic admixture with Europeans has been less than half a percent per generation.

Jewish law tracing back almost 2,000 years states that Jewish affiliation is determined by maternal ancestry, so the Y chromosome study addresses the question of how much non-Jewish men may have contributed to Jewish genetic diversity.

Dr. Hammer was surprised to find how little that contribution was.

"It could be that wherever Jews were, they were very much isolated," he said. The close genetic affinity between Jews and Arabs, at least by the Y chromosome yardstick, is reflected in the Genesis account of how Abraham fathered Ishmael by his wife's maid Hagar and, when Sarah was then able to conceive, Isaac. Although Muslims have a different version of the story, they regard Abraham and Ishmael, or Ismail, as patriarchs just as Jews do Abraham and Isaac.

- Nicholas Wade, New York Times May 2000

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