It's not often that cultural and religious persecution makes countries more diverse, but the Spanish Inquisition might have done just that.
One in five Spaniards and Portuguese has a Jewish ancestor, while a tenth of Iberians boast North African ancestors, finds new research.
This melting pot probably occurred after centuries of coexistence and tolerance among Muslims, Jews and Christians ended in 1492, when Catholic monarchs converted or expelled the Islamic population, called Moriscos. Sephardic Jews, whose Iberian roots extend to the first century AD, received much the same treatment.
"They were given a choice: convert, go, or die," says Mark Jobling, a geneticist at the University of Leicester, UK. Some of those that became Christian would have ended up contributing genes to the Iberian pool.
Historians know that some Sephardic Jews – Conversos or Cristãos Novos – integrated into Spanish and Portuguese society, while previous genetic studies have uncovered North African heritage in a small proportion of Iberians.
But to get a better gauge on intermarriage and integration, Jobling and colleagues in Spain, Portugal and Israel analysed the DNA of more than a thousand Spanish and Portuguese men who don't consider themselves Jewish or Islamic.
Focusing on the Y-chromosome, which passes from father to son, Jobling's team matched each Iberian's DNA to modern Basques, North Africans, or Sephardic Jews. They chose Basques as representative of Iberians who trace their ancestry to early Europeans because studies have shown they interbred sparingly with other groups.
"You're taking modern populations as proxies for past populations," Jobling says.
Overall, 19.8% of Iberian men boasted Y-chromosomes that seemed to descend from a Sephardic Jew, while 10.6% of men had a North African male ancestor, Jobling's team found.
Some areas were more mixed than others, though. In Catalonia, a region in northeast Spain, few men had Jewish or Muslim ancestors. High proportions of men from Galicia and Castille in northwest Spain boast a North African heritage. In nearby Asturias, numbers of men with Sephardic Jewish Y-chromosomes equal those with European chromosomes.
These findings probably hold true for women as well: "Normally migration is only male, but later on we know there must have been some more wholesale migration," says Jobling.
A previous study of maternally inherited mitochondrial DNA put North African heritage at 8 to 9% for Iberian women. Female Sephardic Jewish descendants remain harder to pin down. Historians have estimated total numbers in Iberia in the tens to hundreds of thousands, with numbers of descendants of Cristãos Novos equally sketchy.
Stephen Oppenheimer, of Oxford University and author of Origins of the British, calls the paper's data "a tremendous addition". However, he says much earlier migrations, 5000 to 10,000 years ago, from the Eastern Mediterranean might confound Sephardic estimates.
"They are really assuming that they are looking at his migration of Jewish immigrants, but the same lineages could have been introduced in the Neolithic," he says.
However, Chris Tyler-Smith, an expert on Y-chromosome genetics at the Sanger Institute in Hinxton, UK, says that slight individual differences in Y-chromosome markers suggest that Iberians and Sephardic Jews share ancestry more recent than several millennia.
"What's exciting about this is the way genetics is now starting to get at real details of history," he says. "10, 15 years ago, geneticists were looking at population samples of 30 or 50 individuals and seeing the broad outlines of human history."
Journal reference: American Journal of Human Genetics (vol 83)
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