A new look at the DNA of the Ashkenazi Jewish population has thrown light on its still mysterious origins.

Until now, it had been widely assumed by geneticists that the Ashkenazi communities of Northern and Central Europe were founded by men who came from the Middle East, perhaps as traders, and by the women from each local population whom they took as wives and converted to Judaism.

But the new study, published online this week in The American Journal of Human Genetics, suggests that the men and their wives migrated to Europe together.

The researchers, Doron Behar and Karl Skorecki of the Technion and Ramban Medical Center in Haifa, and colleagues elsewhere, report that just four women, who may have lived 2,000 to 3,000 years ago, are the ancestors of 40 percent of Ashkenazis alive today. The Technion team's analysis was based on mitochondrial DNA, a genetic element that is separate from the genes held in the cell's nucleus and that is inherited only through the female line. Because of mutations -- the switch of one DNA unit for another -- that build up on the mitochondrial DNA, people can be assigned to branches that are defined by which mutations they carry.