More than five centuries after the expulsion and forced conversion of Spanish and Portuguese Jewry, the results of a new genetic study might just spur a return of historic proportions to Israel and the Jewish people. In a paper published in the latest issue of the American Journal of Human Genetics, a team of biologists dropped a DNA bombshell, declaring that 20% of the population of Iberia has Sephardic Jewish ancestry. Since the combined populations of Spain and Portugal exceed 50 million, that means more than 10 million Spaniards and Portuguese are descendants of Jews. These are not the wild-eyed speculations of a newspaper columnist, but rather cold, hard results straight out of a petri dish in a laboratory. The study, led by Mark Jobling of the University of Leicester in England and Francesc Calafell of the Pompeu Fabra University in Barcelona, analyzed the Y chromosomes of Sephardim in communities where Jews had migrated after the expulsion from Spain in 1492. Their chromosomal signatures were then compared with the Y chromosomes of more than 1,000 men living throughout Spain and Portugal. Since the Y chromosome is passed from father to son, the geneticists were able to measure the two groups up against each other, leading to the remarkable finding that one-fifth of Iberians are of Jewish descent. THIS RESULT underlines the extent to which our ancestors suffered so long ago in Spain and Portugal. From the historical record, we know that as early as 1391, a century before the expulsion of the Jews from Spain, widespread anti-Semitic pogroms swept across the country, leaving thousands dead and many communities devastated. In the decades that followed, there were waves of forced conversions as part of an increasingly hostile and dangerous environment for Jews. This reached a climax in 1492, when King Ferdinand and Queen Isabella gave Spain's remaining Jews a dire choice: convert or leave forever. Large numbers chose exile. American historian Howard Morley Sachar has estimated the number of Spain's Jewish exiles at around 100,000, while Hebrew University's Haim Beinart has put the total at 200,000. Others have spoken of even more. But untold numbers of forcibly converted Jews, as well as those who voluntarily underwent baptism, remained. THESE INCLUDE, of course, the Anousim (Hebrew for "those who were coerced"), many of whom bravely continued to cling to Jewish practice, covertly passing down their heritage from generation to generation. In recent years, a growing number of Anousim from across Europe, South America and parts of the US have begun to return to Israel and the Jewish people. But what makes the findings of the genetic study so important is that they attest to the Spanish monarchs' terrible success in subjugating their Jewish subjects and compelling the bulk of those forced to convert to eventually assimilate into the Catholic majority. For centuries thereafter, the ruthless arm of the Inquisition hunted down and killed suspected "Judaizers" or "secret Jews," ultimately forcing many to abandon the faith to which they had remained so heroically, and secretly, loyal. According to the late historian Cecil Roth, the Inquisition's henchmen murdered more than 30,000 "secret Jews." Some were burned alive in front of cheering crowds, while countless others were condemned for preserving Jewish practices. It is no wonder, then, that many of them eventually succumbed to despair and seemingly disappeared as Jews. Until now, that is. THE FINDING that 20% of the population of Iberia is descended from Jews will likely take Spain and Portugal by storm. The results, as The New York Times put it last Friday, "provide new and explicit evidence of the mass conversions of Sephardic Jews" which took place over 500 years ago on Spanish and Portuguese soil. It is the biological equivalent of the pintele Yid, the eternal and unbreakable Jewish spark that can never be extinguished. Indeed, it is as if a large mirror were suddenly being held up in front of every Spanish and Portuguese person, forcing them to look at themselves and see the reality of their national, and individual, history. But even more compelling than what it says about the past is what it might just say about the future. If Israel and the Jewish people undertake a concerted outreach effort toward our genetic brethren in Iberia, it could have a profound impact in a variety of fields, ranging from anti-Semitism in Europe to the future of Jewish demography. Imagine if just 5% or even 10% of Spanish and Portuguese descendants of Jews were to return to Judaism. It would mean an additional 500,000 to 1 million Jews in the world. And even if many or most choose not to return, it still behooves us to reach out to them. The very fact that such large numbers of Spaniards and Portuguese have Jewish ancestry could have a significant impact on their attitudes toward Jews and Israel, possibly dampening their anti-Semitism and anti-Israel slant. For when someone discovers they are of Jewish descent, it is likely to create a greater sense of kinship for Jewish causes. Hence, we should seek to promote and cultivate their affinity for Israel and the Jewish people. Moreover, I believe we have a historical responsibility to reach out to the descendants of the victims of the forced conversions and the Inquisition, and to facilitate their return. Through no fault of their own, their ancestors were cruelly taken from us. Centuries ago, the Catholic Church devoted enormous resources to tearing them away from the Jewish people, and it nearly succeeded. Our task now should be to show the same level of determination to welcome them back into our midst. The writer is the founder and chairman of Shavei Israel (www.shavei.org), which assists Anousim in Spain, Portugal and South America to return to the Jewish people.