I cropped the image above from the paper Inference of Population Structure using Dense Haplotype Data. The main reason was emphasize the distinctiveness of the Sardinian cluster, on the bottom right. As you can see this population exhibits a lot of coancestry across individuals. This isn’t too surprising, Sardinia is an island, and islands are often genetically distinctive. Random genetic drift prevents populations from diverging through gene flow, but water is a major impediment to gradual isolation by distance dynamics. The original Sardinians are naturally going to diverge from mainlanders over time, and begin to share the same set of common ancestors in the recent past, because their space of reasonable mating possibilities is constrained. The other population which is similar in the heat map above are the residents of the Orkneys, off the north coast of Scotland (the Orkneys has a much smaller population than Sardinia, but, it is also much closer to the mainland).
This is on my mind because Dienekes has a long post where he explores the D-statistic results of various European populations, using Sardinians as one of the references. You don’t need to know the details, just that Northern European populations seem to exhibit an affinity to East Asians. Our favorite human genomicsts at Broad highlighted this tendency, and Dienekes went looking (if David Reich et al. were inclined toward mischief they should just posit some crazy scenario, and see if Dienekes assembles the requisite data set!). Which is all fine, and we’ll see more of this in the near future. It isn’t as if others aren’t using Sardinians.
In terms of autosomal DNA, the Iceman clearly clusters with modern Sardinians, and also appears slightly more removed than them compared to continental Europeans. Interestingly, at least as far as the PC analyssi shows, Sardinians appear to be intermediate between the Iceman and SW Europeans, rather than Italians. Perhaps, this makes sense if the Paleo-Sardinian language is indeed related to languages of Iberia.
This trend aroused a little curiosity in me too. I’m sure Dienekes & company will be probing these issues a lot in the near future, but I couldn’t wait. I took the IBS data set, which includes a lot of individuals from various areas of Spain, the Sardinians, French and French Basque from the HGDP, and the Tuscans from the HapMap, and threw them together into a pot. I added HGDP Russians & Orcadians (the latter a British group) to make sure there was a North European “outgroup.” In terms of technical details the combined data set had ~220,000 SNPs, not too shabby. Additionally, I decided to run a PCA, where this number of SNPs is more than sufficient.
On a technical note, the Sardinians were swamped in raw numbers by Iberians and Tuscans (over 100 and around 80 respectively). This means that the peculiarities of the Sardinian genetic heritage didn’t show up, rather, what you see are the Sardinians as they arrange themselves in relation to the genetic variation of these more numerous groups. I used SmartPCA to generate the 10 largest independent dimensions of variation. To make a long story short there really wasn’t much variation added from the second dimension on in this relatively homogeneous sample. So below is PC 1 and 2 (E1 and E2).
Well, the paper is finally out, New insights into the Tyrolean Iceman’s origin and phenotype as inferred by whole-genome sequencing. In case you don’t know, Ötzi the Iceman died 5,300 years ago in the alpine region bordering Austria and Italy. His seems to have been killed. And due to various coincidences his body was also very well preserved. This means that enough tissue remained that researchers have been able to amplify his DNA. And now they’ve sequenced it enough to the point where they can make some inferences about his phenotypic characteristics, and, his phylogenetic relationships to modern populations.
The guts of this paper will not be particularly surprising to close readers of this weblog. The guesses of some readers based on what the researchers hinted were correct: Ötzi seems to resemble mostly closely the people of Sardinia. This is rather interesting. One reason is prosaic. The HGDP sample used in the paper has many Northern Italians (from Bergamo). Why is it that Ötzi does not resemble the people from the region that he was indigenous to? (we know that he was indigenous because of the ratio of isotopes in his body) A more abstruse issue is that it is interesting that Sardinians have remained moored to their genetic past, enough so that a 5,300 year old individual clearly can exhibit affinities with them. The distinctiveness of Sardinians jumps out at you when you analyze genetic data sets. They were clearly set apart in L. L. Cavalli-Sforza’s The History and Geography of Human Genes, 20 years ago. One reason that Sardinians may be distinctive is that Sardinia is an isolated island. Islands experience reduced gene flow because they’re surrounded by water. And sure enough, Sardinians are especially similar to each other in relation to other European populations.
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.”