Published in the journal Science, it analysed the genomes of 271 inhabitants of the peninsula from different historical periods and has contrasted them with data collected in previous studies of 1,107 ancient and 2,862 modern subjects. The results show an unprecedented image of the transformation of the Iberian population along different historical and prehistoric stages.
Replacement of the male population in the Bronze Age
The arrival of groups descended from shepherds of the steppes of Eastern Europe between 4,000 and 4,500 years ago involved the replacement of approximately 40% of the local population and almost 100% of men. "The genetic results are very clear in this regard. Progressively over a period that may have lasted some 400 years, the lineages of the Y chromosome present until then in Copper Age Iberia were almost entirely replaced by a lineage, R1b-M269, of steppe ascendency", says researcher Carles Lalueza-Fox of the Institute of Evolutionary Biology (CSIC and Universitat Pompeu Fabra joint centre).
"While this was clearly a dramatic process, the genetic data alone cannot tell us what prompted it", says David Reich, principal investigator at Harvard Medical School, jointly in charge of the study.
"It would be wrong to state that the local population was displaced, since there is no evidence of violence in that period", adds Iñigo Olalde, a Harvard University researcher.
"An alternative explanation would be that the local Iberian women preferred the newcomers from Central Europe in a context of "strong social stratification", states Lalueza-Fox.
The research team highlights that the genetic data alone cannot reveal the whole story. "The evidence from other fields, such as archaeology and anthropology, must be combined with these results to better understand what prompted this genetic pattern", concludes Reich.
As an example of this replacement phenomenon, the study documents a tomb found at a Bronze Age site (after the Copper Age) in the town of Castillejo del Bonete (Ciudad Real). Of the two individuals found at the burial site, the man descends from the steppe while the woman is genetically similar to the Iberian population prior to the Copper Age.
Another major finding of the study is that the genetics of today's Basques has hardly changed since the Iron Age (about 3,000 years ago). Contrary to some theories that placed the Basques as the descendants of Mesolithic hunters or of the first farmers who lived in the Iberian Peninsula, the results of this work show that the genetic influence of the steppes also reached the Basque Country (in fact they have one of the highest frequencies of the Y chromosome R1b). By contrast, they show hardly any influences of later migrations such as the Romans, Greeks or Muslims, from which they were isolated.
"It is now thought that Indo-European languages spread across Europe thanks to the descendants of the peoples of the steppes. In this study we recompose the complex genetic mosaic of the Iberian Peninsula, where we find Indo-European-type paleolanguages, such as Celtiberian, and non-Indo-European ones, such as Euskera, which is the only pre-Indo-European language still spoken. Our results indicate a greater genetic component of the steppes in Celtiberians than in Iberians, but in any case there is some dissociation between language and ancestry", adds Lalueza-Fox.
The distribution of the genetic flow from Africa to the peninsula is much older than hitherto documented. The study confirms the presence in the centre of the Iberian Peninsula, at the site of Camino de las Yeseras (Madrid), of an individual from North Africa who lived about 4,000 years ago, and a grandson of an African emigrant at a site in Cádiz from the same era. Both men bore considerable proportions of sub-Saharan ancestry. However, they were sporadic contacts that left a scarce genetic mark on the Iberian populations of the Copper and the Bronze Ages.
Moreover, the results indicate that there was a North African flow of genes in the southeast of the Peninsula in Punic and Roman times, long before the arrival in the Peninsula of the Muslims in the eighth century.
Romans, Greeks, Phoenicians, Visigoths and Muslims
The analysis of the genetic map shows profound changes in population in the Iberian Peninsula in more recent historical periods. "For the first time, we document the genetic impact of the major events in the history of the peninsula. The results show that by the start of the Middle Ages at least a quarter of the Iberian ancestry had been replaced by new population flows coming from the eastern Mediterranean (Romans, Greeks and Phoenicians), which reveals that migrations during this period still had a major influence on the formation of the Mediterranean population", explains Íñigo Olalde.
One example of this phenomenon cited in the work is the Greek colony of Empúries, in north-eastern Spain, founded between 600 BC and the late Roman period. The 24 individuals analysed are divided into two groups of different genetic heritage: one composed of individuals with typical Greek ancestry and another composed of a population that is genetically indistinguishable from the Iberians of the nearby village of Ullastret.
"The article also discusses the arrival in the Peninsula of the Visigoths and the Muslims. Among the former, two individuals have been located at the site of Pla de l'Horta (Girona) with clear Eastern European ancestry and mitochondrial DNA typical of Asia. From the Islamic period, individuals of Granada, Valencia, Castellón and Vinaròs have been analysed showing a North African component of close to 50%, far higher than the residual 5% observed in the current Iberian population. This ancestry was practically eliminated during the Reconquista and the subsequent expulsion of the Moors", comments Lalueza-Fox.
Population structure of Mesolithic Iberia
This study, together with other one published the same day on Current Biology, identifies for the first time the presence of a spatial and temporal genetic structure among the hunter-gatherers of the Iberian Peninsula during the Mesolithic (about 8,000 years ago). In the northwest, the Mesolithic hunters who lived a few centuries before the arrival of the first farmers show a genetic affinity with central European hunter-gatherers. This ancestry was not present in the previous hunter-gatherers from the same region or in the contemporary hunter-gatherers in south-eastern Iberia in the late Mesolithic.
The research has been funded by "La Caixa", FEDER-MINECO (BFU2015-64699-1118P), the National Institute of Health (grant GM100233), the Paul G. Allen Family Foundation, and the Howard Hughes Medical Institute, among others.