Read: Slavery and the Jews

In the case of conversos, DNA is helping elucidate a story with few historical records. Spain did not allow converts or their recent descendants to go to its colonies, so they traveled secretly under falsified documents. “For obvious reasons, conversos were not eager to identify as conversos,” says David Graizbord, a professor of Judaic studies at the University of Arizona. The designation applied not just to converts but also to their descendants who were always Catholic. It came with more than a whiff of a stigma. “It was to say you come from Jews and you may not be a genuine Christian,” says Graizbord. Conversos who aspired to high offices in the Church or military often tried to fake their ancestry.

The genetic record now suggests that conversos—or people who shared ancestry with them—came to the Americas in disproportionate numbers. For conversos persecuted at home, the fast-growing colonies of the New World may have seemed like an opportunity and an escape. But the Spanish Inquisition reached into the colonies, too. Those found guilty of observing Jewish practices in Mexico, for example, were burned at the stake.

Chacón-Duque and his colleagues pieced together the genetic record by sampling DNA from 6,500 people across Brazil, Chile, Colombia, Mexico, and Peru, which they compared to that of 2,300 people all over the world. Nearly a quarter of the Latin Americans shared 5 percent or more of their ancestry with people living in North Africa and the eastern Mediterranean, including self-identified Sephardic Jews. DNA alone cannot prove that conversos were the source of this ancestry, but it fits with the historical record. This pattern of widespread but low North African and eastern Mediterranean ancestry in the population suggests that its source is centuries old, putting the date around the early days of New Spain. In contrast, more recent immigration to Latin America from Italy and Germany in the late 19th century shows up concentrated in relatively few people in a few geographic areas.

Geneticists have also noticed rare genetic diseases prevalent in Jews popping up in Latin America. “It’s not just one disease. It’s like, wow, this isn’t a coincidence,” says Harry Ostrer, a geneticist at Albert Einstein College of Medicine. In 2011, Ostrer and his colleagues decided to study two populations—in Ecuador and Colorado—with unusually high prevalence of two mutations often found in Jews. (One mutation was in the breast-cancer gene BRCA1, and the other caused a form of dwarfism called Laron syndrome.) And indeed, they found enriched Sephardic Jewish ancestry in the 53 people they tested. With advances in DNA technology, Chacón-Duque and his colleagues were able to carry out similar research, but on the scale of thousands of people.