May 9, 2000
Y Chromosome Bears Witness to Story of the Jewish Diaspora
By NICHOLAS WADE
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.
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
"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.