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Genetic Ancestry of Iberian Peninsula Population Shifted Over Time

NEW YORK (GenomeWeb) – Researchers have delved into the genomic history of the people of the Iberian Peninsula in two new studies, tracing shifts in ancestry over time.

Iberia, home to present-day Spain and Portugal, is thought to differ in its genetic ancestry from the rest of Europe due to its climate and location on the western edge of the continent. For example, it may have remained hospitable during the last Ice Age, enabling it to have potentially acted as a refuge for people who fled south. Later, its proximity to North Africa may have allowed for different genetic influences.

Using ancient DNA collected from samples dating back 13,000 years ago as well as samples from more recent eras, two research teams examined the genetic ancestry of Iberian Peninsula peoples. One study, focusing on Paleolithic hunter-gatherers and Neolithic farmers, found that rather than the latter replacing the former, the hunter-gatherers already shared some ancestry with the arriving group and mixed with them. The second team, meanwhile, took a broader look to show how various groups moved into the region to leave their genetic mark, including a complete replacement of local Y chromosome lineages during the Bronze Age and, later, an increase in Steppe genetic ancestry.

"The signs of mixing between local hunter-gatherers and newly arriving farmers is fantastic to witness, even though we have virtually no overlap in the radiocarbon record in many areas of Iberia," Vanessa Villalba, a PhD student at the Max Planck Institute for the Science of Human History and the first author of one of the studies, said in a statement. "It just shows how much of the past we are still missing."

Previous work has indicated that Western and Central European populations were dominated first by ancestry similar to that of an approximately 14,000-year-old individual from Villabruna, Italy, but that this ancestry was then mostly replaced by one more similar to 19,000- to 15,000-year-old individuals linked to the Magdalenian culture.

Villalba and her colleagues generated autosomal and mitochondrial DNA sequences from 11 new samples from around Spain and Portugal, dating back to the Paleolithic, Mesolithic, and Neolithic, which they then compared to previously published data from other ancient and modern-day individuals. As they reported today in Current Biology, they found that all Iberian hunter-gatherers harbored ancestry from both the Villabruna- and Magdalenian-related individuals.

This, the researchers said, suggests that two Late Pleistocene lineages were able to persist in Iberia and that there was a possible early connection between these two refuges. Additionally, as this ancestry persisted in Iberia, the researchers said it indicates that hunter-gatherers in the region mixed with farmers.

Meanwhile, researchers led by Harvard Medical School's David Reich generated genome-wide SNP data on 271 ancient Iberians from the Mesolithic, Neolithic, Bronze, Copper, Iron, and more recent ages. They merged this with data from more than 1,100 other ancient and 2,862 present-day individuals.

Similar to the Max Planck-led team, Reich and his colleagues also noted that Iberian hunter-gatherers contributed to the genetic makeup of farmers arriving in the region from Anatolia. They also found that by about 2,000 BC, about 40 percent of Iberian ancestry and nearly all its Y-chromosome ancestry was replaced by ancestry originating from the Steppes, as they reported in Science.

Why this Y-chromosomal replacement occurred isn't clear, the researchers said, noting that additional archaeological and anthropological research was needed.

This influx of Steppe ancestry, they found, did not always correspond with a linguistic shift to Indo-European languages. Present-day Basque individuals, for example, genetically resemble Iron Age Iberian populations, though speak a non-Indo-European language.

During the Roman period and thereafter, the ancestry of the Iberian Peninsula was shaped by gene flow from North Africa and the eastern Mediterranean, they added.

In an accompanying commentary in Science, the University of Cambridge's Marc Vander Linden acknowledged the friction between ancient DNA studies and archaeological and anthropological research, as genetic research can destroy samples and sometimes lacks the perspective the other fields provide regarding cultural history, but said the approaches could be combined.

"There have been tensions, but the opportunity is there for a transdisciplinary approach to population history and a better understanding of the role of human behavior in shaping gene flow," he wrote.