Genetic history of Spain and Portugal's peoples

The gene pool of Spaniards and Portuguese is overwhelmingly Mediterranean, but it retains clear regional differences between north and south, and between east and west. Those differences come from millennia of migrations and a settlement pattern shaped by geography.

The Iberian Peninsula is a puzzle of mountains and valleys that for millennia worked as a machine for producing differences. Before modern transport shortened distances, each valley could live with its back to the next one, and that shows in culture, in language and also in genes. Spain and Portugal, however, rank among Europe's oldest countries with continuous political unity, and that long coexistence within each state encouraged internal intermarriage far earlier than in Italy or Germany, with a gently homogenising effect on each country's gene pool.

## A dark-skinned hunter from Asturias

The genetics of today's Iberians is overwhelmingly Mediterranean. But the sequencing of the genome of a **7,000-year-old** hunter-gatherer found at **La Braña**, in Asturias, produced a result that breaks that intuition: that individual shared genetic affinities much closer to today's northeastern Europeans than to today's Iberians themselves, and he also had dark skin. In just a few millennia, the peninsula's genetic landscape changed from top to bottom.

It is worth not drawing conclusions from a single genome. Hunter-gatherers from North Africa may have crossed the Strait of Gibraltar by boat and colonised the southern part of the peninsula, while hunter-gatherers from northern and central Europe occupied northern Spain. The full picture of what all Mesolithic Iberians looked like remains unfinished.

## Two Neolithic routes and an already divided peninsula

The Neolithic farmers did not arrive from a single place either. A first Mediterranean route brought farmers of the Cardial Ware culture from the Balkans and Italy. Shortly afterwards, the **La Almagra** pottery culture developed in Andalusia, which appears to emerge from the present-day Maghreb. That episode would explain the presence of DNA from northwestern Africa and the Red Sea across much of southern and western Iberia, with markers such as the Y haplogroups **E-M81**, **J1** and **T**.

From very early on, then, Spain was genetically divided between north and south, and between east and west. Homogenisation would begin with the Reconquista, when northerners recolonised the south, and would continue in the trinc centuries with slow but constant intermarriage, especially in the cities. Even so, the regional genetic landscape remains very marked, on both the maternal and paternal sides.

## Phoenicians, Celts, Goths and Berbers: each left their mark

Phoenicians, Celts, Greeks, Jews, Romans, Goths, Suebi, Franks, Arabs and Berbers all passed through the peninsula. All of them left a genetic imprint on the populations of the regions where they settled. Three tools are used to trace it: Y-chromosome haplogroups, which are passed almost unchanged from father to son; mitochondrial DNA, which is inherited only from the mother; and whole-genome studies.

Each haplogroup corresponds to a distinct ancestral lineage and is subdivided into a phylogenetic tree, which is simply a family tree of genetic ancestry. The mountainous geography explains why those marks survive unevenly: one valley may show a signature that the neighbouring valley has all but erased.

## What Iberian genetics does not say

Genetic data describe lineages and mixtures, not national identities or rights over a territory. A marker being more frequent in the south than in the north does not make anyone the heir to anything. Population genetics works with frequencies and probabilities, and its results should be read with that caution.