A team of geneticists studying the ancestry of Jewish communities has found an unusual genetic signature that occurs in more than half of Levites of Ashkenazi descent.

The signature is thought to have originated in Central Asia -- not in the Near East, the ancestral home of Jews.

The finding raises the question of how the signature became so widespread among the Levites, an ancient caste of hereditary Jewish priests, whose lineage supposedly dated back to a son of Jacob.

The genetic signature occurs on the male or Y chromosome and comes from a few men, or perhaps a single ancestor, who lived about 1,000 years ago, just as the Ashkenazi community was beginning to be established in Europe.

Most American Jews are descended from Ashkenazis, one of the two main branches of the Jewish community. The other is the Sephardis, whose ancestors were expelled from Spain.

The new report, published in the current issue of the American Journal of Human Genetics, was prepared by population geneticists in Israel, the United States and England, who have been studying the genetics of Jewish communities for the last six years.

They say 52 percent of Levites of Ashkenazi origin have a particular genetic signature that originated in Central Asia, although it is also found in low frequency in the Middle East.

The ancestor who introduced it into the Ashkenazi Levites perhaps could have been from the Khazars, a Turkic tribe whose king converted to Judaism in the eighth or ninth century, the researchers suggest.

Their reasoning is that the signature, a particular set of DNA variations known as R1a1, is relatively common in the region north of Georgia that was once occupied by the Khazar kingdom.

If the patrilineal descent of the two priestly castes -- Cohanim and Levites -- had been followed as tradition describes, then all Cohanim should be descended from Aaron, the brother of Moses; and all Levites from Levi, the third son of the patriarch Jacob.

Scientists found that more than half the Cohanim, in both the Ashkenazi and Sephardi communities, indeed carried the same genetic signature on their Y chromosome. Their ancestor lived some 3,000 years ago, based on genetic calculations, and indeed might have been Aaron, says Karl Skorecki of the Technion and Rambam Medical Center in Haifa.

But the picture among the Levites was less clear, suggesting they had a mixed ancestry.