Y-chromosomal Aaron
Y-chromosomal Aaron is the name given to the hypothesised ancestor of the Kohanim (singular "Kohen", "Cohen", or Kohane), a patrilineal priestly caste in Judaism. In Scripture, this ancestor is identified as Aaron, the brother of Moses.
The Cohen Modal Haplotype, which can be identified with a genealogical DNA test, is:
DYS19/DYS394 = 14 DYS388 = 16 DYS390 = 23 DYS391 = 10 DYS392 = 11 DYS393 = 12
The Cohen Modal Haplotype belongs to haplogroup J.
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 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 a Y chromosome inherited from their father.
Males who share a common patrilineal 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 remains unchanged except for accumulated mutations).
The hypothesis
Membership in the Jewish community is traditionally passed via the maternal line (see Who is a Jew). Membership in the specific group that makes up the Jewish priesthood (the Kohens or Kohanim), 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.
It is important to note that, simply as a result of statistics, for any population there will be a common Y chromosome ancestor, whether or not the population is even of the same race; the hypothesis that all Kohanim share the same Y chromosomal ancestor will consequently always prove true, even if they are not all descended from the biblical Aaron. The more useful hypothesis to test is whether all the Kohanim are more closely related, in Y chromosome terms, than individual Kohanim are to other groups, i.e. is the Y chromosomal Aaron fairly exclusive to the Kohanim, or is the ancestor also an ancestor of many other semitic groups, e.g. Arabs.
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.
Lemba
The Cohen Modal Haplotype has also been found in the Lemba of Southern Africa, who have a tradition of Jewish ancestry[1].
Kurds
The Cohen Modal Haplotype has also been found in some groups of Kurds. There are a number of Kurdish Jews, though the Haplotype's presence could also indicate that Y chromosomal Aaron is an earlier ancestor, and thus that all Kohanim are not necessarily descended from a single 12th century Israelite named 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[2], but also leading to criticism[3].
Other carriers of the DNA
Critics of the theory point out that the Cohen Modal Haplotype has also been found in significant numbers in groups of non-Jews, notably Italians. However propeonents of the theory are quick to explain these anomolies. They state that history records the migration of large numbers of Jewish slaves to Rome (what is now Italy), particularly in association with the building of the Colosseum.
Y-chromosomal Levi?
While Kohanim are believed to have descended in the patrilineal line from Aaron, brother of Moses, Levites (a second level of Jewish priesthood) are traditionally 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 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 within the last 2000 years for many Levites of the Ashkenazi community.[4]
Founder effect
People who do not believe in the validity of the Cohen Modal Haplotype say that such efforts to detect a genetic signature's survival in the Y chromosome unchanged over 3,500 years (approximately the amount of time since the time of the patriarchs) seem misplaced. While it has been shown that surname studies and Y-DNA matches validate each other[5], the most current research increasingly points to a relatively recent historical timeframe for founder effects and matching haplotypes.
According to a study by a forensic team[6], the average mutation rate for Y-STRs (the "alleles" that create differences in haplotype scores) is 2.1 x 10(-1), or once every 476 generations, i.e., once every 9,520 years. But this estimate is computed on the basis of one allele changing – on its either increasing by 1 or its diminishing by 1. Where most people err is to forget that these statistics apply to one microsatellite.
Hence, even if Aaron's progeny did prosper and survive over the millennia, they would have split into dozens of different genealogies. Some believe that if the discoverers of the "Cohen Modal Haplotype"[7] found any modal Jewish haplotype at all it is probably the genes of a prolific medieval rabbi who lived about 1000 years ago and had numerous sons and grandsons. Others argue that the fact that so many people with a family belief of being Cohainim have tested positive for the Cohen Modal Haplotype offers proof of its validity. The former conclusion consorts better with the emerging consensus that the roots of the human family tree are much shallower than previously believed. However, mutation rates are bitterly disputed and only longitudinal studies such as comparisons available to large DNA surname project administrators will yield secure estimates.
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.
External links
- A summary of Jewish & Kohanim genetic research papers
- Origin, Diffusion, and Differentiation of Y-Chromosome Haplogroups E and J: Inferences on the Neolithization of Europe and Later Migratory Events in the Mediterranean Area
- The Y chromosome pool of Jews as part of the genetic landscape of the Middle East by Nebel et al.