Planet Money recently did a report on the difficulty of maintaining high economic productivity in southern Italy. I won’t rehash the specifics of the story, but, I think it is important to get a visual sense of just how large the contrast between the south and north of Italy is. Too often we speak of nation-states. Nation-states are real, and they are important, but they are often not comparable. Just like comparing the USA to Sweden is only marginally informative, so comparing a small nation like Ireland to a more substantial one like Italy is deceptive. Here is a 2008 regional GDP map with sub-national breakdowns. Though some of the values are certainly lower now (basically, everything outside of Germany and Sweden), the relationships still hold.
Early this year I received an email from Dr. Peter Ralph, inquiring if I might discuss some interesting statistical genetic results from analyses of the POPRES data set which might have historical relevance. I’ve been excitingly waiting for the preprint to be made public so it could trigger some wider discussion. I believe that the methods outlined in the paper perhaps show us a path into the near future, where we might gain a much sharper perspective upon the recent past. So it’s finally out, and you can read it in full. Ralph and Dr. Graham Coop have posted put it up at arXiv, The geography of recent genetic ancestry across Europe. The paper uses ~500,000 SNPs from the POPRES data set individuals, and looks at patterns of identity by descent as a function of geography. By identity by descent, we’re talking about segments of the genome which are derived from a common ancestor. Because of recombination the length of the segments can give us a sense of the date of the last common ancestor; long segments indicate more recent ancestry because fewer recombination events have chopped up sequence.
Here’s the big takeaway of the paper: …There is substantial regional variation in the number of shared genetic ancestors: especially high numbers of common ancestors between many eastern populations likely date to the Slavic and/or Hunnic expansions, while much lower levels of common ancestry in the Italian and Iberian peninsulas may indicate weaker demographic effects of Germanic expansions into these areas and/or more stably structured populations. Recent shared ancestry in modern Europeans is ubiquitous, and clearly shows the impact of both small-scale migration and large historical events….
A new paper in Science has just been published which in its broad outlines has been described in conference presentations. When examining the autosomal genetic variation of three individuals of the hunter-gatherer Pitted Ware Culture (PWC), and one of the agriculturalist Funnel Beaker Culture (TRB), the authors found that the two groups were sharply differentiated. The number of SNPs was on the order of 10,000 or so if I read the methods correctly. This is rather thin for studying contemporary within European population differences (~100,000 or more seems to be safe), in particular using hypothesis based clustering algorithms (it seems more manageable for PCA). But the findings are strong enough that I think we shouldn’t discount them. The most fascinating aspect of the results is that while the PWC seem to exhibit affinities with Northern and Northeastern Europeans, the TRB individual seems more similar to extant Southern Europeans!
Others have already commented extensively on the results. Keeping in mind the small sample sizes, limitation of comparisons, and the relatively thin marker set, I think the primary result we can take away from these findings is that old models of pure cultural and demographic diffusion are false. By this, I mean that prior debates which culminated in the early aughts on the “Paleolithic vs. Neolithic” contribution to the ancestry of modern Europeans were fundamentally premised on a demographic diffusion dynamic, whereby genes and ideas exhibited a continuous flow across a flat and featureless landscape. On the contrary, the basic outlines we are seeing here is that the human past exhibited spatial and temporal discontinuity. And why should this surprise us? There is no dialect continuum between Spanish and Chinese across Eurasia. Rather, broad language families are sharply differentiated from each other at zones of contact. Though there are theoretical reasons why the variation in genes should be more clinal, the reality remains that cultural parameters are going to shape the outlines of genetic variation, and those parameters are discontinuous.
The image above come from John Hawks’ weblog. I was thinking today about the resettlement of Europe since the Last Glacial Maximum. It is clear that much of northern Europe was not habitable until the Holocene, after the Ice Age. And those regions which were habitable were often marginal. But, there were zones of southern Europe which remained relatively clement. One model of how Europe was settled after the warming is that hunter-gatherers expanded north out of these southern refuges. This can explain the lower heterozygosity of northern populations (see map to left). They may have lost their genetic diversity to some extent through population bottlenecks or simply drift on the wave of demographic advance. And yet something jumped out at me on this map: the southwest portion of Portugal is reputedly the zone with the highest African admixture in continental Europe (for historical reasons). The heterozygosity may simply be a function then of the fact that southern Europe has been in greater contact with other regions of the world because of geographic proximity.
If I had to guess, I would propose that most extant Europeans will be discovered to be a 2-way West Asian/Ancestral European mix, just as most South Asians are a simple West Asian/Ancestral South Indian mix. In both cases, the indigenous component is no longer in existence and the South Asian/Atlantic_Baltic components that emerge in ADMIXTURE analyses represent a composite of the aboriginal component with the introduced West Asian one. And, like in India, some populations will be discovered to be “off-cline” by admixture with different elements: in Europe these will be Paleo-Mediterraneans like the Iceman, an element maximally preserved in modern Sardinians, as well as the East Eurasian-influenced populations at the North-Eastern side of the continent.
This does not seem to be totally implausible on the face of it. But it seems likely that any “West Asian” component is going to be much closer genetically to an “Ancestral European” mix than they were to “Ancestral South Indians,” because the two former elements are probably part of a broader West Eurasian diversification which post-dates the separation of those groups from Southern and Eastern Eurasians. In other words, pulling out the distinct elements in Europeans is likely a more difficult task because the constituents of the mixture resemble each other quite a bit when compared to “Ancestral North Indians” vs. “Ancestral South Indians.”
I decided to take the Dodecad ADMIXTURE results at K = 10, and redo some of the bar plots, as well as some scatter plots relating the different ancestral components by population. Don’t try to pick out fine-grained details, see what jumps out in a gestalt fashion. I removed most of the non-European populations to focus on Western Europeans, with a few outgroups for reference.
Here’s a table of the correlations (I bolded the ones I thought were interesting):
|W Asian||NW African||S Europe||NE Asian||SW Asian||E Asian||N European||W African||E African||S Asian|
Ole Magga, Norwegian politician
On this blog I regularly get questions about the Sami (Lapp*). That’s because I often talk about Finnish genetics, have readers such as Clark who are of part-Sami origin, and, the provenance and character of the Sami speak to broader questions about the emergence of the modern European gene pool. More precisely questions about the Sami are relevant to the broader nature of the Finnic presence in Europe, and their relationship to other Baltic and northern populations. Are these people “indigenous” to Europe, or relatively newcomers (prehistoric Magyar or Turks).? These questions are prompted by the peculiarity of their languages (as well as the physical appearance of some of the Sami). With Basque they are the only living non-Indo-European European languages whose origins are prehistoric (Magyar and Turkish were arrivals within the last 1,000 years).**
Because of affinities to other Uralic languages which are found in Central Siberia it has often been conjectured that the Finns, Sami, and Estonians are relative newcomers to Norden from that region. This has some equivocal support from Y chromosomal lineages. On the other hand, there are those who argue that the Finnic peoples were present in the north of Europe before the arrival of Indo-European speakers (often these are Finnish nationalists). This has some support from maternal lineages. Naturally, some have been tempted to synthesize these two genetic lines of evidence, and the linguistic affinities, to argue that Finns are a hybrid population of Asiatic men and Paleolithic European women! But we need to go further than uniparental markers, the direct male and female ancestral lines. We need to look across the broader swath of the genome. It just happens that a new paper was published in The European Journal of Human Genetics on autosomal Sami affinities to other populations, A genome-wide analysis of population structure in the Finnish Saami with implications for genetic association studies: