Historians have long searched for clues to the Jewish past, but attention is now shifting to science, namely genetics, that might offer a tool to unlock Jewish history.

Danny Villareal always thought he came from a venerable old Catholic family in Mexico that had come north 300 years ago when Texas was still part of the Spanish colonial empire.

It was only after researching the mineral rights associated with his family’s original land grants that the faded documents led him to an intriguing discovery: His ancestors might have been Conversos—forced Jewish converts originally from Spain who fled to these remote areas to avoid the Inquisition. “Our family always joked about having Jewish blood,” he said, but nobody took it seriously.

It was not until Villareal, a partner in an architectural firm, signed up for a personal DNA test four years ago that he knew for certain about his Jewish origins. It motivated him to start an online group for others bearing the Villareal family name throughout the Americas. About 25 other Villareals have since been tested. Almost all carry DNA indicating a Semitic origin commonly found among Jews, and a few have since joined mainstream Jewish life.

Giving legitimacy to converso claims is only a small fraction of the impact that ongoing research into DNA—the genetic markers that provide a biological pathway to our ancestors—is having on our knowledge of Jewish genealogy and the Jewish past.

According to Dr. Harry Ostrer, director of the Human Genetics Program in the Department of Pediatrics at New York University Medical Center and a leading figure in Jewish DNA studies, current research is falling into three distinct categories: migratory patterns, diseases and genealogy.

Arguably the best-known application is the ability to verify whether a Jewish male is a kohen, a member of the priestly class descended from Aaron, the brother of Moses. Markers distinguishing the Levites, the other priestly class descended from Levi, have also been identified.

When it comes to the female line—in the case of Ashkenazim—there are two competing theories about genetic origins. One has it that many Jewish women in diaspora communities probably started out as local non-Jews who became so protective of their adopted Jewish identity that they made sure marriages never moved outside the core group. “Later, entry into the community, relative to its size, was much less,” said Neil Bradman, Ph.D., chairman of the Center for Genetic Anthropology at University College, London, who has looked into this issue.

Initially, the husbands were most likely itinerant Jewish traders who settled in a town for commercial reasons—for example, port cities that rimmed the Mediterranean—and then sought marriage partners from the area. Historians have long believed that many of these communities may have developed even before the end of the biblical period.

Bradman has summarized much of the current research in a chapter called “Threads to Antiquity,” published in the 2004 anthology Traces of Ancestry: Studies in Honour of Colin Renfew (McDonald Institute for Archaeological Research). He intends to explore the topic more extensively in an upcoming book.

The second theory suggests “founding mothers”—four Jewish women who may have also lived as early as the biblical period and became the progenitors of close to half of Ashkenazim living today. This theory, put forward this spring in the American Journal of Human Genetics by a research team from the Technion and Rambam Medical Center in Haifa, implies that at some point Jewish men settled abroad with families rather than acquiring wives locally.

A separate result concerns differences between Ashkenazi Levites and North African and Mizrahi Levites. Ashkenazi Levites differ in that they absorbed one or more non-Jewish contributors to their gene pool about 1,000 years ago, according to a 2003 study led by Dr. Doron M. Behar, who is also connected to the Technion and Rambam Medical Center in Haifa.

These Ashkenazi men may have come from a priestly class within the Khazars—a Turkic tribe of Central Asia that converted to Judaism around the 9th century. The mutation is not found among Middle Eastern and North African Levites.

DNA studies have also had an impact on competing theories concerning the absorption of these Khazars into the general Ashkenazi population, an issue raised 30 years ago by Arthur Koestler in The Thirteenth Tribe (Random House). He argued that the Khazars contributed heavily to the gene pool of today’s Ashkenazi Jews and were responsible for the explosion in their numbers over the past 500 years, relative to other Jews.

This hypothesis is now under revision. The Khazar input seems closer to a mere 9 percent, said Kevin Brook, an independent historian from Danbury, Connecticut. Brook is synthesizing current DNA research into an updated edition of his 1999 book, The Jews of Khazaria (Jason Aronson). His Web site, www. khazaria. com, summarizes a wide range of Jewish DNA research (and has become an excellent source for all sorts of Jewish DNA topics).

Another issue that dna research is helping to resolve is the geographic origins of Ashkenazim. It is now believed that they are probably descended from Italian Jews of late antiquity (2nd to 5th centuries). Dr. Ostrer found a close correlation between contemporary Italian Jews still living in Rome who trace their ancestors back 2,000 years and Ashkenazim. He is working on a book that will delve deeper into current research.

Moreover, a 1998 study, reported in the International Journal of Legal Medicine, similarly found a kohen marker among the non-Jewish Italians of central and southern Italy. It helped confirm a strong Jewish presence in the area during Roman times when Jews, particularly men, would often marry outside the religious community.

DNA tests have also shown that the people closest to Jews from a genetic viewpoint appear to be the Kurds. This finding was reported in a 2001 study from the Hebrew University of Jerusalem that was published in the American Journal of Human Genetics, a discovery that could corroborate the biblical account that Abraham originally came from the Fertile Crescent.

Genetic research into so-called Jewish diseases—those more likely to be carried by people of Jewish origin—is helping to verify what has long been suspected but never proven. For example, the presence of pemphigus, a genetic blistering disease, occurring almost equally among today’s Spaniards and Jews, points to shared genetic stock. This could be due to the mass conversions of Spanish Jews to Christianity in the 15th century, according to a study conducted by a team of Israeli and Spanish scientists. This finding was reported in the April 2004 issue of Tissue Antigens, a medical journal.

The growing ability to attach a Jewish component to certain genetic diseases—whether from Eastern Europe or the Mediterranean—is another benefit of DNA technology. Dr. Ostrer said a list that includes prostate and breast cancer, schizophrenia, Tay-Sachs and familial Mediterranean fever should help alert more Jews to the value of testing for a predisposition. He cites in particular Gaucher Disease—half of whose victims are Ashkenazim—and the enzyme-replacement therapy to control it, which has “been one of the great success stories” in treating the ailment.

An outburst of criticism was ignited last year when the Journal of Bioscience published a paper by Henry Harpending, an evolutionary anthropologist at the University of Utah, and Gregory Cochran, an independent scholar, theorizing that diseases such as Gaucher helped Ashkenazim become smarter than the surrounding population. Included as evidence was a study showing the increased neural growth in animals with Gaucher and a higher-than-normal IQ among patients at the Gaucher Clinic at the Shaare Zedek Medical Center in Jerusalem. Debate is still raging.

Sometimes the most intriguing findings can emerge from an analysis of one’s own DNA—an opportunity that’s only been available to the public for about six years.

Consider the story of Cezary Fudali, a lawyer in his thirties who was originally from Poland but now lives in Ottawa. A few years ago, he learned that his mother—born during World War II—had been adopted as an infant, apparently after her adoptive parents had seen a baby girl being carried around a railroad station by a woman in desperate circumstances. “Perhaps this meant my mother was Jewish or gypsy,” said Fudali. His family could not tell him more.

He decided that a DNA screening might be a way to uncover his genealogy. The tests indicated that his mother came from predominantly Jewish roots. “It has now become my story and it’s changing me,” he said. “I am more interested in learning about Jewish culture. I was interested in the Holocaust before, but now much more so.”

Companies handling jewish clients include Family Tree DNA of Houston (www.familytreedna.com); DNA Print Genomics of Sarasota, Florida (www.dnaprint.com); and Trace Genetics of Richmond, California (www.tracegenetics.com), which recently merged with Print Genomics.

The cost for a test is around $100 to $200. Typically, the company will send a small test kit by mail. The client then takes a cheek scraper (provided) and scrapes the inside of the mouth a few times at intervals of several hours, sending the samples back to the testing company in the packing provided. The results are usually delivered several weeks later, either by mail or online.

Family tree dna is considered to have the largest database of comparative samples and surnames. Bennett Greenspan, its president, said that DNA testing can tell you if you share a common ancestor with someone carrying the same family name. It can tell if you are a male descendent of a kohen or a Levite. And most recently, with its newest samples from Mediterranean Jews, it can possibly tell if your ancestors originally came from countries like Spain, even if they ended up as Ashkenazim in Eastern Europe.

And it is easier, he noted, to find matches for a man because the markers for women have mutated at a slower rate and are thus harder to track. Women’s DNA also seems to have spread out more widely, he added, possibly because they were married off to Jewish men in other villages while their brothers stayed home to run the family enterprise. But be careful, warned Greenspan. “We come across people who are sure they are part of a certain family but their DNA suggests they are not,” as happened to Cezary Fudali. Others might be descendents of an extramarital affair that led to a false paternity, he added.

Even when the person is certain he or she is of Jewish origin, the results can be surprising. That is what happened to Alanya Snyder, a teacher in Oakland, California. All four of her grandparents hailed from Eastern Europe, so she assumed she carried the most commonly held Jewish DNA markers. But three years ago, when she was offered a test as a wedding present by childhood friend Jason Eshleman, a partner in Trace Genetics, she learned a different story.

Snyder discovered she belonged instead to the genetic grouping from Central Asia. This placed her among the small percentage of Jews most likely descended from Turkic Khazars or other Asians who centuries ago may have married merchant Jews traveling the Silk Road.

“It’s fascinating,” she said.

Andrée Aelion Brooks’s most recent book on Jewish history is Russian Dance (John Wiley & Sons). She can be reached atandreebrooks@hotmail.com.