The Bronze Age Indo-European invasion of Europe

Last summer, I was eagerly awaiting the publication of the genome of the Tyrolean Iceman. It is quite remarkable that only a year later, there is now autosomal DNA from half a dozen prehistoric Europeans. By comparing their DNA to that of modern populations, we are beginning to understand how the current mosaic of European peoples was formed.

The ancient samples vary greatly in the number of SNPs tested, and we cannot be sure how well they map to the restricted range of modern populations. Nonetheless, a crystal clear general pattern seems to emerge, at least in its broadest outlines (Sources: Oetzi the Iceman, Neolithic Swedes, Mesolithic Iberians):

We can plainly see that European hunter-gatherers best map to the modern Atlantic_Baltic population component. This is well represented in the remotest areas of Europe, the ones most distant from the Near Eastern womb of nations. It can be reasonably supposed that the modern Atlantic_Baltic component partially captures alleles present in the ancient European hunter-gatherers, the mtDNA haplogroup U population that seems to stretch from Iberia to Siberia. (However, note, that this does not mean that the Atlantic_Baltic component represents hunter-gatherer ancestry only.)

Conversely, ancient European farmers also possess a large chunk of the Southern component which is absent in the hunter-gatherers. This occurs at high frequencies today around the Mediterranean and reaches its maximum in the Near East. It is clear that there is direct evidence that farming came to Europe not as an idea, but as a people, just as archaeology and physical anthropology had always indicated -- until the rather modern distaste for migration set in.

But there is another component present in modern Europe, the West_Asian which is conspicuous in its absence in all the ancient samples so far. This component reaches its highest occurrence in the highlands of West Asia, from Anatolia and the Caucasus all the way to the Indian subcontinent. It is well represented in modern Europeans, reaching its minima in the Iberian peninsula, Sardinia, and Finland. A sampling of populations, including those closest geographically to the ancient samples:

There are several observations we can make:

• The West_Asian component has a pan-European distribution: it must have been involved in a pan-European process rather than a more localized historical phenomenon.
• Its absence from prehistoric individuals down to ~5ky ago suggests that it may have been added to the European population at a later date, although it may already have been present in currently unsampled areas (e.g., the Balkans) prior to 5kya.
• It reaches its lowest occurrence in areas where non-Indo-European languages have been spoken (Basques and Iberia in general, Sardinia, and Finland)

The post-5kya timeframe is also conventionally accepted by linguists for either the dispersal of Indo-European languages, or at least of a significant subset thereof.

In that respect, it is important to note the correspondence between the West_Asian autosomal component and the k5 component of Metspalu et al. (2011): the latter is the major West Eurasian element in the Indian subcontinent. If a major episode of West Eurasian admixture took place in India 1,200-4,000 years ago, and keeping in mind uncertainties about dating, it may very well be that this corresponds, at least in part, to the eastern manifestation of the same phenomenon.

The Bronze Age is an important transitional phase in European archaeology: distinctive archaeological cultures with distinctive physical types make their appearance across the continent. There appears to be substantial innovation in metallurgy, weapons and transportation, increase in raiding, abandonment of settlements, and formation of broad-range confederacies with distinctive archaeological markers.

I propose that a quantum leap in social complexity occurred during this period. Remarkably, after ~4,000YBP, there are no longer farmers and hunter-gatherers as distinct cultures anywhere in Europe, and their mtDNA gene pools begin to expand in synch with each other. It may very well be that climatic upheavals framing this period may have triggered population movements, both Indo-European and Semitic.


Perhaps, through a combination of better technology and social organization, the Indo-European speaking nucleus, originally one among many linguistic groups of the prehistoric Near East were able to transmit their language, culture, and ideology to much larger populations, by alternatively subjugating or incorporating them. We can thus view the Indo-European bearers as "creative destructors", upsetting the balance of established societies and re-creating them in their own image.

Both the wide differences in genetic composition among present-day Indo-European speakers, and their early-attested physical contrasts testify to the fact that the original IE nucleus did not maintain itself apart --at least not for long!-- from the populations they encountered; in this they appear different from the earlier farmers who apparently kept their Mediterranean affinity even in the northernmost edge of their expansion, thousands of years after their entry into Europe.

Nonetheless, some of the legacy of the earliest Indo-European speakers does appear to persist down to the present day in the genomes of their linguistic descendants, and I predict that when we sample later (post 5-4kya) individuals we will finally find the West_Asian piece that is missing from the European puzzle.

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ADDENDUM:

On the right are the K12b results for the ancient individuals, which further suggest that the Mesolithic Europeans possessed a Northern European affinity that is lacking in the farmer individuals. Conversely, farmer individuals have an excess of the Atlantic_Med component, and also a presence of the Southwest_Asian one.

These results are compatible with the above discussion, as well as the observation by Sánchez-Quinto et al. (2012) that:

the position of La Braña individuals in the 1000 Genomes Project data and the 1KGPomnichip PCAs suggests that the uniform Mesolithic substrate could be related to modern Northern European populations but may represent a gene pool that is no longer present in contemporary Southern European populations.

I wrote a small survey a couple years ago, in which I argued that:

I will simply say that the recent results of Bramanti et al. for a U-dominated older mtDNA stratum in Central/North-eastern Europe can be reasonably extended to cover both North-western Europe and northern Eurasia up to Lake Baikal, the prehistoric limit between Caucasoids and Mongoloids.

The extension of U dominance to Iberia, as well as the detection of the North European autosomal signal in both Iberia as well as, presumably, Siberia, seems to make the case for the existence of this postulated zone pretty clear.

One could very well call this boreal population Ancestral Northwest Eurasians (ANWEA) and parallelize it with the Ancestral South Indians (ASI). These Paleolithic substrata frame the Eurasian landmass on opposite latitudinal ends, and were the receptacles of the great chain of migrations which began in the Neolithic womb of nations. One of the final episodes of this process was the dispersal of the Indo-European languages during the Copper and Bronze Ages.

UPDATE (July 3): Indo-European genetic signatures in an Orcadian genome and a Lithuanian and Ancient European DNA using DIYHarappaWorld and MDLP5