The largest study to date of ancient DNA from the Iberian Peninsula offers new insights into the populations that lived in this region over the last 8,000 years. The most surprising discovery suggests that the local (male) Y chromosomes were almost completely replaced during the Bronze Age.
From 2500 BC. and for some 500 years, analysis indicates tumultuous social events that shaped the paternal ancestry of Iberians to the present day.
The work, published in Science on March 15 by a 111-person international team led by researchers from Harvard Medical School and the Barcelona Institute of Evolutionary Biology, also details genetic variation among ancient hunter-gatherers, documents interbreeding between ancient Iberians with the peoples of North Africa and the Mediterranean, and provides a further explanation of why today's Basques, who have such a distinctive language and culture, are also ancestrally different from other Iberians.
Some of the finds support or clarify what is known about the history and prehistory of Iberia, while others challenge it.
The team analyzed genomes from 403 ancient Iberians who lived between 6000 B.C. and 1600 AD, 975 people from outside the Iberian Peninsula and about 2,900 today.
The researchers found that as early as 2500 BC, Iberians began to live alongside people who migrated from central Europe and had recent genetic ancestors in the Russian steppes. Within a few hundred years, analyzes showed that the two groups had interbred extensively.
For example, at the Bronze Age site known as Castillejo de Bonete in Spain, where a woman and a man were found buried side by side, analysis revealed that the woman's ancestry was entirely local, while the man had ancestors from central Europe.
According to the researchers, the men and women of the two groups contributed strikingly unequal proportions of DNA to subsequent generations.
Before the arrival of the Central Europeans, the Iberians had no detectable recent ancestry outside the Iberian Peninsula. After 2000 BC, 40 percent of the ancestors of Iberians in general and 100 percent of their patrilineal ancestors - that is, their father and their father's father, etc. - can be traced back to groups that arrived from Central Europe.
According to Carles Lalueza-Fox, principal investigator of the Paleogenomics Laboratory of the Institute of Evolutionary Biology and co-lead author of the study, the data suggest that there was an important genetic change that is not obvious in the archaeological record.
It is not yet clear what could have caused such a dramatic change. The researchers believe that it would be a mistake to conclude that the Iberian men were forcibly killed or displaced, since the archaeological record does not give clear evidence of an outbreak of violence in this period.
An alternative possibility is that local Iberian women preferred the newcomers from Central Europe in a context of strong social stratification, says Lalueza-Fox.
As the centuries passed, the paternal ancestry continued to evolve. However, most Iberian men today can trace their paternal ancestry back to these latecomers from the Bronze Age.
The hunter-gatherer groups scattered throughout the Iberian Peninsula had a very different genetic makeup from each other in the Mesolithic era, from about 8000 B.C. to 5500 BC, suggesting that new hunter-gatherer groups migrated to the Iberian Peninsula and transformed local populations before farmers arrived with their own ancestors from Anatolia. An independent study on Iberian hunter-gatherers, published in Current Biology the same day as the Science article , reaches similar conclusions.
A person buried in Iberia between 2400 B.C. and the year 2000 B.C. had entirely North African ancestry, and a second person who lived between 2000 B.C. and the year 1600 B.C. He had a grandfather of North African descent. The new study confirms that migrations also occurred from Africa to Europe.
Samples of Iron Age people who lived around 900 B.C. to 19 BC in areas where very different languages were spoken, they had significant ancestry from the Russian steppe. This suggests that the influx of people did not always cause linguistic changes, specifically the adoption of Indo-European languages.
Today's Basques are genetically similar to those of the Iron Age throughout the Iberian Peninsula, leading researchers to hypothesize that Basque ancestry and language remained relatively intact for millennia, while other groups their surroundings blended and changed more significantly.
North African ancestry was more widespread in Iberia during the Roman period (around 20 BC to 400 AD) than previously believed, especially in the south. The genetic influence is appreciated long before groups from North Africa conquered Iberia during the 8th century AD.