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Y-chromosomal Aaron is the name given to the hypothesized most recent common ancestor of many of the patrilineal Jewish priestly caste known as Kohanim (singular "Kohen", "Cohen", or Kohane). In the Torah, this ancestor is identified as Aaron, the brother of Moses. The hypothetical most recent common ancestor was therefore dubbed "Y-chromosomal Aaron".
The original scientific research was based on the hypothesis that a majority of present-day Jewish Kohanim share a pattern of values for 6 Y-STR markers, which researchers named the Cohen Modal Haplotype (CMH).
Additional early research using 12 Y-STR markers indicated that about half of contemporary Jewish Kohanim shared Y-chromosomal J1 M267, specifically haplogroup J-P58 (also called J1c3). Other Kohanim groups share a different ancestry, including haplogroup J2a (J-M410).
Although membership in the Jewish community has, since at least the second century CE, been passed maternally (see: Who is a Jew?), tribal identity, and membership in the group that originally comprised the Jewish priesthood (Cohen or Kohen; plural: Cohanim or Kohanim), has been patrilineal. Modern Kohanim claim descent from a biblical person, Aaron, brother of Moses, in the direct lineage from Levi, the patriarch of the Tribe of Levi, great grandson of Abraham, according to the tradition codified in the Tanakh (שמות / Sh'mot/Exodus 6). DNA testing is aiding scholars to trace the lineages found among modern Jewish populations, including contemporary Cohen families, to decipher origins of the people groups who were joined to the ancient Israelites and to identify genetic admixture and genetic drift.
For human beings, the normal number of chromosomes is 46, of which 23 are inherited from each parent. Two chromosomes, the X chromosome and Y chromosome, determine sex. 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 the Y chromosome inherited from their father.
Males who share a common patrilineal ancestor also share a Y chromosome, diverging only with respect to accumulated mutations. Since Y-chromosomes are passed from father to son, all Kohanim men should theoretically have almost identical Y chromosomes; this can be tested with a genealogical DNA test. As the rate that mutations accumulate on the Y chromosome is relatively constant, scientists can estimate the elapsed time since two men had a common ancestor. (See molecular clock.)
The Cohen hypothesis was first tested by Prof. Karl Skorecki and collaborators from Haifa, Israel, in 1997. 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 Cohens, pointing to a common Cohen population origin before the Jewish diaspora under the Roman empire. However, this study also indicated that only 48% of Ashkenazi Cohanim and 58% of Sephardic Cohanim have the J1 Cohen Modal Haplotype.
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 Cohen STR results clustered around a single pattern they named the Cohen Modal Haplotype:
xDE xDE,PR Hg J CMH.1 CMH CMH.1/HgJ CMH/HgJ Ashkenazi Cohanim (AC): 98.5% 96% 87% 69% 45% 79% 52% Sephardic Cohanim (SC): 100% 88% 75% 61% 56% 81% 75% Ashkenazi Jews (AI): 82% 62% 37% 15% 13% 40% 35% Sephardic Jews (SI): 85% 63% 37% 14% 10% 38% 27%
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-Cohen Jew of Haplogroup J.
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.
Following the discovery of the very high prevalence of 6/6 CMH matches amongst Cohens, others were quick to look for it, and often to see it as a signpost for possible Jewish ancestry, though the chromosome itself is not exclusive to Jews.
News of 6/6 matches in the Lemba of Southern Africa were seen as confirming a possible Jewish lineage (Thomas MG et al. 2000); and some suggested that 4/4 matches in non-Jewish Italians might be a genetic inheritance from Jewish slaves, deported by Emperor Titus in large numbers after the fall of the Temple in AD 70, some of whom were put to work building the Colosseum in Rome.
However, there was also criticism that the paper's evidence was being overstated.
One source of early confusion was the low resolution and general coverage of the Cohen Modal Haplotype. The Cohen Modal Haplotype (CMH), while frequent amongst Cohens, also appeared in the general populations of haplogroups J1 and J2 with no particular link to the Cohen ancestry. These haplogroups occur widely throughout the Middle East and beyond. Thus, while many Cohens have haplotypes close to the CMH, a greater number of such haplotypes worldwide belong to people with no likely Cohen connection at all.
Individuals with at least 5/6 matches for the original 6 marker Cohen Modal Haplotype occur widely across the Middle East, with significant frequencies in various Arab populations mainly with J1 Haplogroup, "that are not traditionally considered admixed with mainstream Jewish populations" – notably Yemen (34.2%), Oman (22.8%), Negev (21.9%), and Iraq (19.2%); and amongst Muslim Kurds (22.1%), Bedouins (21.9%), and Armenians (12.7%).
On the other hand, Jewish populations were found to have a "markedly higher" proportion of full 6/6 matches, according to the same (2005) meta-analysis, compared to these non-Jewish populations, where "individuals matching at only 5/6 markers are most commonly observed".
The authors nevertheless warned that "using the current CMH definition to a infer relation of individuals or groups to the Cohen or ancient Hebrew populations would produce many false-positive results," and note that "it is possible that the originally defined CMH represents a slight permutation of a more general Middle Eastern type that was established early on in the population prior to the divergence of haplogroup J. Under such conditions, parallel convergence in divergent clades to the same STR haplotype would be possible."
Y-DNA patterns from around the Gulf of Oman were analysed in more detail by Cadenas et al. in 2007. The detailed data confirm that the main cluster of haplogroup J1 haplotypes from the Yemen appears to be some genetic distance different from the CMH-12 pattern typical of Eastern European Ashkenazi Cohens.
Even within the Jewish Cohen population, it became clear that there were multiple different Cohen lineages, including distinctive lineages both in Haplogroup J1 and in haplogroup J2. It also turned out that there were other unrelated groups of Jewish lineages in Haplogroup J2 which matched the original 6-marker CMH, but which were not associated with Cohens. Current estimates based on the accumulation of SNP mutations place the defining mutations that distinguish haplogroups J1 and J2 as having occurred about 20 to 30,000 years ago.
In response to the low resolution of the original 6-marker CMH, the testing company FTDNA released a 12-marker CMH signature that was more specific to the large closely related group of Cohens in Haplogroup J1.
A further academic study published in 2009 examined more STR markers and identified a more sharply defined SNP haplogroup, J1e* (now J1c3, also called J-P58*) for the J1 lineage. The research found "that 46.1% of Kohanim carry Y chromosomes belonging to a single paternal lineage (J-P58*) that likely originated in the Near East well before the dispersal of Jewish groups in the Diaspora. Support for a Near Eastern origin of this lineage comes from its high frequency in our sample of Bedouins, Yemenis (67%), and Jordanians (55%) and its precipitous drop in frequency as one moves away from Saudi Arabia and the Near East (Fig. 4). Moreover, there is a striking contrast between the relatively high frequency of J-58* in Jewish populations (»20%) and Kohanim (»46%) and its vanishingly low frequency in our sample of non- Jewish populations that hosted Jewish diaspora communities outside of the Near East."
Recent phylogenetic research for haplogroup J-M267 placed the "Extended Cohen Modal Haplotype" in a subhaplogroup of J-L862,L147.1 (age estimate 5631-6778yBP yBP): YSC235>PF4847/CTS11741>YSC234>ZS241>ZS227>Z18271 (age estimate 2731yBP).
Cohens in other haplogroups
Behar's 2003 data point to the following Haplogroup distribution for Ashkenazi Cohens (AC) and Sephardic Cohens (SC) as a whole:
Hg: E3b G2c H I1b J K2 Q R1a1 R1b Total AC 3 0 1 0 67 2 0 1 2 76 4% 1½% 88% 2½% 1½% 2½% 100% SC 3 1 0 1 52 2 2 3 4 68 4½% 1½% 1½% 76% 3% 3% 4½% 6% 100%
The detailed breakdown by 6-marker haplotype (the paper's online-only table B) suggests that at least some of these groups (e.g. E3b, R1b) contain more than one distinct Cohen lineage. It is possible that other lineages may also exist, but were not captured in the sample.
Hammer et al. (2009) identified Cohanim from diverse backgrounds, having in all 21 differing Y chromosome haplogroups: E-M78, E-M123, G-M285, G-P15, G-M377, H-M69, I-M253, J-P58, J-M172*, J-M410*, J-M67, J-M68, J-M318, J-M12, L-M20, Q-M378, R-M17, R-P25*, R-M269, R-M124 AND T-M70.
Similar investigation was made with men who consider themselves Levites. Whereas the priestly Kohanim are considered by those who believe in the historicity of Aaron to be descendants of Aaron, who in turn was a descendant of Levi, son of Jacob, the Levites (a lower rank of the Temple) are considered descendants of Levi through other lineages. Levites should also therefore share common Y-chromosomal DNA.
The investigation of Levites found high frequencies of multiple distinct markers, suggestive of multiple origins for the majority of non-Aaronid Levite families. One marker, however, present in more than 50% of Eastern European (Ashkenazi) Jewish Levites points to a common male ancestor or very few male ancestors within the last 2000 years for many Levites of the Ashkenazi community. This common ancestor belonged to the haplogroup R1a1 which is typical of Eastern Europeans or West Asians, rather than the haplogroup J of the Cohen modal haplotype, and most likely lived at the time of the Ashkenazi settlement in Eastern Europe, and thus was not really a Levite. However, a study published in the journal Nature in November 2013 disputes this and suggests that because of the absence of the M582 mutation among Eastern Europeans, this marker most likely originates in a pre-Diasporic population in the Near East.
The E1b1b1 haplogroup (formerly known as E3b1) has been observed in all Jewish groups worldwide. It is considered to be the second most prevalent haplogroup among the Jewish population outside of the J haplogroups. According to one non-peer reviewed paper it has also been observed in moderate numbers among individuals from Ashkenazi, Sephardic and Samaritan communities having traditions of descending from the tribe of Levi, suggesting that the E1b1b1 men claiming to be Levites may have existed in Israel before the Diaspora of 70 CE.
The Samaritan community in the Middle East survives as a distinct religious and cultural sect and constitutes one of the oldest and smallest ethnic minorities in the world, numbering just less than 700 members. As a religious sect, reportedly, the Samaritans broke away from the mainstream Jews around the fifth century BCE, but according to Samaritan accounts it was the southern tribes that left the original worship set forth by Joshua. The Samaritans have maintained their religion and history to this day. Samaritans claim to descend from the Biblical Israelite tribes of Ephraim, Menashe and Levi.
Since the Samaritans maintain extensive and detailed genealogical records for the past 13–15 generations (approximately 400 years) and further back, it is possible to construct accurate pedigrees and specific maternal and paternal lineages. Y-Chromosome studies have shown that the majority of Samaritans belong to haplogroups J1 and J2, while the Samaritan Kohanim belong to haplogroup E1b1b1a (formerly known as E3b1a).
"The Samaritan M267 lineages differed from the classical Cohen modal haplotype at DYS391, carrying 11 rather than 10 repeats", as well as, have a completely different haplogroup, which should have been "J1". Samaritan Kohanim descend from a different patrilineal family line, having haplogroup E1b1b1a (M78) (formerly E3b1a).
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Confusingly, because only 4 of the markers that Malaspina et al. tested were markers in common with the CMH study, 3 of which matched, they originally concluded that all of the CMH matches should be identified with what is now called Haplogroup J2. This is now known not to be the case.
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