Y-chromosomal Aaron is the hypothesized most recent common ancestor of the patrilineal Jewish priestly caste known as Kohanim (singular Kohen, also spelled Cohen). According to the traditional understanding of the Hebrew Bible, this ancestor was Aaron, the brother of Moses. Historical-critical reading of the biblical text suggests that the origin of the priesthood could have been much more complex, and that for much if not all of the First Temple period, Kohen may have not (necessarily) been synonymous with "Aaronide". Rather, this traditional identity seems to have been adopted sometime around the Second Temple period. The original scientific research was based on the hypothesis that a majority of present-day Jewish Kohanim share a pattern of values for six Y-STR markers, which researchers named the extended Cohen Modal Haplotype (CMH). Subsequent research using twelve Y-STR markers indicated that nearly half of contemporary Jewish Kohanim shared Y-chromosomal J1 M267 (specifically haplogroup J-P58, also called J1c3), while other Kohanim share a different ancestry, such as haplogroup J2a (J-M410). While these genetic studies were seen as possibly supporting the traditional biblical narrative, subsequent research (by the original researchers and others) has challenged this conclusion in a number of ways and has in fact shown that the genealogical record "refutes the idea of a single founder for Jewish Cohanim who lived in Biblical times", and rather "seems to vindicate the historical-critical hypothesis of competing priestly clans."
Background
For human beings, the normal number of chromosomes is 46, of which 23 are inherited from each parent. Two chromosomes, the X and Y, determine sex. Females have two X chromosomes, one inherited from each of their parents. Males have an X chromosome inherited from their mother, and a Y chromosome inherited from their father. Males who share a common patrilineal ancestor also share a common Y chromosome, diverging only with respect to accumulated mutations. Since Y-chromosomes are passed from father to son, all Kohanim men should theoretically have nearly identical Y chromosomes; this can be assessed with a genealogical DNA test. As the mutation rate on the Y chromosome is relatively constant, scientists can estimate the elapsed time since two men had a common ancestor. Membership in the Jewish Kohanim caste has been determined by patrilineal descent (see Presumption of priestly descent), which would make it possible for the characteristic to be tracked using the Y chromosome passed on to each male descendant. Modern Kohanim are traditionally regarded in Judaism as male descendants of biblical Aaron, the inaugural List of high priests of Israel and brother of Moses. Aaron was a direct patrilineal descendant of Abraham, according to the lineage recorded in the Hebrew Bible (שמות / Sh'mot/Exodus 6). With the development of methods to follow specific DNA sequences of the human genome, interest in the Cohanim (and Levites) has gained new momentum as an instrument for proof of the common origins of the current Jewish ethnic-groups in the population of the Land of Israel two thousand years ago, as narrated in the biblical story. Skorecki, who carried out the initial study, told the journalist Jon Entine, "I was interested in the question: To what extent was our shared oral tradition matched by other evidence?"
Initial study The Kohen hypothesis was first tested through DNA analysis in 1997 by Karl Skorecki and collaborators from Haifa, Israel. In their study, "Y chromosomes of Jewish priests", published in the journal Nature, they found that the Kohanim appeared to share a different probability distribution compared to the rest of the Jewish population for the two Y-chromosome markers they tested (YAP and DYS19). They also found that the probabilities appeared to be shared by both Sephardic and Ashkenazi Kohens, pointing to a common Kohen population origin before the Jewish diaspora at the destruction of the Second Temple. However, this study also indicated that only 48% of Ashkenazi Kohens and 58% of Sephardic Kohens have the J1 Cohen Modal Haplotype. Such genetic markers were also found in approximately 5% of Jews who did not believe themselves to be kohanim. In a subsequent study the next year (Thomas MG et al., 1998), the team increased the number of Y-STR markers tested to six, as well as testing more SNP markers. Again, they found that a clear difference was observable between the Kohanim population and the general Jewish population, with many of the Kohen STR results clustered around a single pattern they named the Kohen Modal Haplotype:
Here, becoming increasingly specific, xDE is the proportion who were not in Haplogroups D or E (from the original paper); xDE,PR is the proportion who were not in haplogroups D, E, P, Q or R; Hg J is the proportion who were in Haplogroup J (from the slightly larger panel studied by Behar et al. (2003)); CMH.1 means "within one marker of the CMH-6"; and CMH is the proportion with a 6/6 match. The final two columns show the conditional proportions for CMH.1 and CMH, given membership of Haplogroup J. The data show that the Kohanim were more than twice as likely to belong to Haplogroup J than the average non-Cohen Jew. Of those who did belong to Haplogroup J, the Kohanim were more than twice as likely to have an STR pattern close to the CMH-6, suggesting a much more recent common ancestry for most of them compared to an average non-Kohen Jew of Haplogroup J.
Dating Thomas, et al. dated the origin of the shared DNA to approximately 3,000 years ago (with variance arising from different generation lengths). The techniques used to find Y-chromosomal Aaron were first popularized in relation to the search for the patrilineal ancestor of all contemporary living humans, Y-chromosomal Adam. Subsequent calculations under the coalescent model for J1 haplotypes bearing the Cohanim motif gave time estimates that place the origin of this genealogy around 6,200 years ago (95% CI: 4.5–8.6 Kybp), earlier than previously thought, and well before the origin of Judaism (David Kingdom, ~2.0 Kybp).
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