A recent study has sequenced the DNA of an ancient Thracian woman but, for some reason, instead of looking at her comparison with modern Bulgarians and such, they have written a study that mostly goes about Ötzi "the iceman" and has not a single Bulgarian sample.
Martin Sikora et al., Population Genomic Analysis of Ancient and Modern Genomes Yields New Insights into the Genetic Ancestry of the Tyrolean Iceman and the Genetic Structure of Europe. PLoS Genetics 2014. Open access → LINK [doi:10.1371/journal.pgen.1004353]
Genome sequencing of the 5,300-year-old mummy of the Tyrolean Iceman, found in 1991 on a glacier near the border of Italy and Austria, has yielded new insights into his origin and relationship to modern European populations. A key finding of that study was an apparent recent common ancestry with individuals from Sardinia, based largely on the Y chromosome haplogroup and common autosomal SNP variation. Here, we compiled and analyzed genomic datasets from both modern and ancient Europeans, including genome sequence data from over 400 Sardinians and two ancient Thracians from Bulgaria, to investigate this result in greater detail and determine its implications for the genetic structure of Neolithic Europe. Using whole-genome sequencing data, we confirm that the Iceman is, indeed, most closely related to Sardinians. Furthermore, we show that this relationship extends to other individuals from cultural contexts associated with the spread of agriculture during the Neolithic transition, in contrast to individuals from a hunter-gatherer context. We hypothesize that this genetic affinity of ancient samples from different parts of Europe with Sardinians represents a common genetic component that was geographically widespread across Europe during the Neolithic, likely related to migrations and population expansions associated with the spread of agriculture.
Notice please that, as the authors acknowledge, the DNA of the second Thracian individual, K8 may be contaminated:
This does not seem to dissuade them to use it in the analyses.
Notice that, instead of attempting to model moderns on ancients, as would seem logical from the viewpoint of purported ancestry but would be incomplete for lack of a sufficiently large ancient sample or allow the ancient samples to "float freely" in the analysis, the researchers decided to force them into modern parameters what is still valid, because it indicates greater or lesser affinity to the various studied modern populations (among which there's not a single Balcanic sample, oddly enough).
We can see that:
- Epipaleolithic Iberian Braña 1 approximates the French structure but is somewhat "more Basque" than these.
- Neolithic Pitted Ware semi-forager Ajv70 (Gotland) approximates the Orcadians very well.
- Neolithic Megalithic/Funnelbeaker Gok4 (Southern Sweden) approximates North Italians.
- Chalcolithic North Italian Ötzi (Iceman) is close to Sardinians but not quite the same ("more Basque" again).
- Iron Age Thracian commoner P 192-1 approximates Tuscans.
- I would ignore princely Thracian K8 because of the aforementioned contamination issues.
For completeness, I'm including here also fig. S1, which includes runs 1-8 of ADMIXTURE:
|Fig S1- ADMIXTURE results for HGDP. Panels show the results for ADMIXTURE runs for k = 2 to k = 8 ancestral clusters on the HGDP individuals, and the corresponding cluster proportions inferred for the ancient samples.|
Notice (see fig. S7) that K=3-5 are quite poor fits and therefore both should be ignored as meaningless. From K=6 onwards the scores slightly improve for all the ancient samples, however it must be said that K=2 is in general the best fit form most European populations, being most of the improvement in error score due to better approximation to West Asian samples.
In most cases Basques have the lowest or one of the lowest fitness scores (except at K=5, where Basques are portrayed as a Russian-Sardinian mix, what is clearly a confounding artifact). Sardinians also have very low error scores but only from K=5 onwards, when the Sardinian component becomes apparent. The Iceman has very low error scores for all K values, while the Thracian samples have the greater ones, maybe owing to the lack of Balcanic samples.
For me these error results suggest that ancients are fine being just "unspecific Europeans" (K=2 blue), while the low error score for Basques and Sardinians surely underline that these are about the only modern populations which can be explained as simple Paleolithic-Neolithic mix, without need of a third Indoeuropean extra ancestry.
They also projected the ancient samples onto PCA plots of modern European populations:
For some odd reason the PCAs are different in each case, even if the samples are the same (only moderns used, ancients are "projected" and should not affect the result). I have no explanation for this issue and I reckon I'm tempted to write to the authors asking for this unexpected complexity, which seems product of the projection itself altering the graph.
Whatever the case, the projection of the ancient samples, follows in general terms the patterns noted above for the ADMIXTURE graph:
- La Braña 1 projects between French and Basques.
- Ajv70 projects onto Orcadians, tending also towards France.
- Ötzi projects between Sardinians and Italians.
- Gok 4 with North Italians but not far from Basques.
- P 192-1 doesn't seem too akin to any specific modern population, although some French, Basques and Tuscans do approximate her.
These results may be frustrating for those already too accustomed to the previous analysis of ancient autosomal DNA but we must not forget that, because of its huge size and complexity, autosomal DNA requires statistical analysis, which is highly susceptible to variations in sample strategy particularly, as well as to other not always well understood factors. Hence different points of view are generally complementary rather that outright contradictory.
Of some interest is also this TreeMix graph of modern populations and Ötzi:
It is notable the African low level admixture arrow at the root of the Euro-Mediterranean branch (the so-called "Basal Eurasian" element in Lazaridis 2013) and the East Asian component in Finns. Also sizable admixture from the West Eurasian root is apparent among Tuscans. Once these admixture axes are allowed for, the topology of the European tree changes significantly, showing a main split between Eastern Europeans (Finns) and Western/Southern ones.
Other similar trees are available in fig. S6.
No extra Neanderthal admixture in Ötzi
Contrary to some previous rough estimates, Ötzi does not appear to differ from other Europeans in Neanderthal ancestry at all. See figs. S9 and S10.