Y-chromosomal Aaron
Y-chromosomal Aaron is the name given to the hypothesised ancestor of the Kohanim (singular "Kohen" or Kohane), a patrilineal priestly caste in Judaism. In Scripture, this ancestor is identified as Aaron, the brother of Moses.
The techniques used to find Y-chromosomal Aaron were first popularized in relation to the search for the patrilineal ancestor of all humans, Y-chromosomal Adam.
Background
In genetics, it is understood that every human has 46 chromosomes, of which 23 are inherited from each parent. Two chromosomes, the X chromosome and Y chromosome, determine gender. Women have two X chromosomes, one inherited from their mother, and one inherited from their father. Men have an X chromosome inherited from their mother, and a Y chromosome inherited from their father.
Males who share a common ancestor should also share a Y chromosome, diverging only with respect to accumulated mutations. Mutations on the Y chromosome occur at a relatively constant rate, allowing scientists to estimate the time that has passed since carriers of similar Y chromosomes had a common ancestor. (See molecular clock).
(Another kind of DNA, Mitochondrial DNA (mtDNA), is not on any chromosome, but is instead passed from mother to child in the cytoplasm of the egg. This always comes from the mother, and also accumulates mutations).
The Hypothesis
Membership in the Jewish community is passed via the maternal line (see Who is a Jew). Membership in the specific group that makes up the Jewish priesthood (the Kohenim), however, is patrilineal. Kohens claim descent from Aaron, brother of Moses. Since male children always get their Y chromosome from their father, it can be hypothesised that all Kohanim should share the same Y chromosome, neglecting any mutations that have occurred since Aaron.
Testing the hypothesis
This hypothesis was first tested by Prof. Karl Skorecki and collaborators from Haifa, Israel. In their 1997 study, they discovered that many Kohanim share certain Y-chromosome markers, which argues in favor of the hypothesis. Other studies (Thomas MG et al, 1998) have corroborated the findings, and have dated the origin of the shared DNA at about 3,000 years ago (with variance arising from different generation lengths). This led to the development of the Cohen Modal Haplotype, a set of Y-chromosomal markers that might have been shared by the Biblical Aaron.
Criticism and Response
Understandably, the finding led to a lot of excitement in religious circles, providing some "proof" of the historical veracity of the Bible [1] (http://www.geocities.com/hrhdavid/English/frames/index.html) [2] (http://www.aish.com/societywork/sciencenature/the_cohanim_-_dna_connection.asp), but also leading to criticism [3] (http://www.ariga.com/genes.shtml).
Was it Jacob?
One might argue that, from the premise of the study, all male Jews should share the same Y chromosome, because, according to the Bible, Aaron and Moses descended patrilineally from Jacob, the ancestor of all Jews. However, religious conversion and out-breeding (by intermarriage or rape) can be expected to have continuously introduced other sources of Y-chromosomal DNA into the Jewish community. Only those who are Kohen and Levite claim a direct male line back to Aaron, and it is in mostly these men that a unique haplotype has been found.
A relatively small number of men in the Jewish community who do not identify as Kohen or Levite have also been found to carry the haplotype. It is possible that this is because they too have a direct line back to Jacob. However, since it is possible for a Kohen to lose the status of Kohen, and since some amount of extra-marital associations can be expected in any community, there are other explanations for the presence of the DNA.
Other carriers of the DNA
The Cohen Modal Haplotype has been also been found in significan numbers in groups of non-Jews, notably Italians, the Lemba of Southern Africa (Thomas MG et al 2000), and some groups of Kurds.
History records the migration of large numbers of Jewish slaves to Rome (what is now Italy), in particular in association with the building of the Colosseum.
Kurds have no direct tradition or history of Jewish lineage, but the area in which most Kurds live (Kurdistan) overlaps the ancient territory of Babylon, for which history records substantial (forced) Jewish immigration.
The Lemba claim they are one of the lost tribes of Israel. DNA evidence now supports this claim.
Y-chromosomal Levi?
While Kohens are believed to have descended in the patrilineal line from Aaron, brother of Moses, Levites (a second level of Jewish priest) are believed to have descended in the patrilineal line from Levi, son of Jacob. Levites should also therefore share common Y-chromosomal DNA.
An investigation of men who consider themselves Levites in fact found two distinct markers. One marker, present in many Eastern European (Ashkenazi) Jewish Levites, and in a majority of Spanish/Mediterranian (Sephardic) Jewish Levites, points to a common male ancestor roughly 3000 years ago. Another marker, however, is present only in Ashkenazi Jewish Levites, and points to a common ancestor about 1000 years ago.
M. G. Thomas, Karl L. Skorecki, H. Ben-Ami, Tudor Parfitt, Neil Bradman, and D. B. Goldstein discuss this discrepancy in "Origins of Old Testament Priests." (pubished in Nature 394, July 9, 1998, 138-140). Their findings are summarized by David Keys, in his book Catastrophe: An Investigation Into the Origins of the Modern World. (New York: Ballantine Books, 2000):
30 percent of Ashkenazi non-Cohenic Levites have a particular combination of DNA material on part of their Y-chromosome that is not shared to any extent by either non-Levite Ashkenazi Jews or the Sephardic community as a whole. This genetic marker does not even show up among the Cohens (descendants of the ancient Israelite Chief Priests) - but only among the descendants of Assistant Priests, and then only within Ashkenazi (northern European) Jewry. What seems to have happened is not only a potentially large-scale conversion of non-Jewish people, almost certainly Khazars, to Judaism, but also the adoption of Levite (Assistant Priest) status by a substantial number of the Khazar converts.... Adoption of Cohenic or ordinary Levitical status by converts was and is expressly forbidden by rabbinical law, so the Khazars had to develop a mythic national history that gave them the right to Levitical status. They claimed that they were the descendants of one of the lost tribes of Israel and were not converts at all but merely returnees to Judaism. Furthermore, the tribe they claimed ancestry from was that of Simeon, the brother of the founder of the tribe of Levi.... Probably it was the old pre-Jewish Khazar priests - the qams - who at the conversion had become Levites en masse..." (excerpts from pages 99-100).
See also
References
- Skorecki K, Selig S, Blazer S, Bradman R, Bradman N, Waburton PJ, Ismajlowicz M, Hammer MF. Y chromosomes of Jewish priests. Nature 1997;385:32. PMID 8985243.
- Thomas MG, Skorecki K, Ben-Ami H, Parfitt T, Bradman N, Goldstein DB. Origins of Old Testament priests. Nature 1998;394:138-40. PMID 9671297.
- Thomas MG, Parfitt T, Weiss DA, Skorecki K, Wilson JF, le Roux M, Bradman N, Goldstein DB. Y chromosomes traveling south: the Cohen modal haplotype and the origins of the Lemba--the "Black Jews of Southern Africa". Am J Hum Genet 2000;66:674-86. PMID 10677325.
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