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Thursday, February 14, 2008

European mtDNA has E-W dominant clines

I was just (re-)reading
Mitochondrial DNA Variation of Modern Tuscans Supports the Near Eastern Origin of Etruscans, by A. Achilli et al. (AJHG, 2007) and stopped in this plot of European and West Asian principal components (mtDNA):

Click image to expand

And it seemed to me a significant, interesting (and quite recent) piece of genetic info to comment about.

It's interesting that there seems to be a quite homogenous cluster including most European populations. The exceptions are:

  1. Most Iberians (except Catalans), that seem to be apart only because of high PC2
  2. Most Italians (except Piamontese and Sardinian), Greeks and Bulgarians, that seem more influenced by what we could call the Neolithic component (extreme in West Asia)
  3. Caucasian peoples that are quite on their own (but near West Asia in the PC1)
But it's even most interesting that for both PC1 and PC2 the extreme examples are in SW Europe and the northern part of West Asia:
  • The PC1 axis could well be assimilable to a line joining the Basque and Iranian samples
  • The PC2 axis is equally comparable to a line running from the Northern Portuguese to the Azeri samples
In other words, both PC ranges span between northern Iberia and the southern Caspian coast. All the rest are intermediate, at least in what regards to PC1 and PC2.

PC3 is also reflected in that plot, using color. Nevertheless most samples are intermediate for it (green). A couple of Greek insular samples are extremely high in PC3 (red), while a handful of unconnected locations are somewhat low (light blue) or very low (deep blue) in it. I can't think of any meaningful info this PC3 may give (but I just love the ingenious way they depicted it).

Back to PC1 and PC2 (here is how haplogroups weight in them), I think it can be concluded that there is a very apparent cline between the Basque Country and Iran (or Northern Portugal and Azerbaijan, almost the same axis) for maternal lineages in West Eurasia. This cline surely reflects the proportion of Neolithic (and maybe also post-Neolithic) influx in Europe. As expected, Greece is quite high (intermediate) in the Neolithic (West Asian) component. More surprising maybe is the case of Italy, not just the south but almost all regions (but curiously not Sardinia).

The vast majority of Europeans instead show a quite less intense degree of "genetic neolithization" but it must be noted that the actual origin of European Neolithic was for the most part in Greece or the Balcans, being less clear the connection between SE Europe and West Asia in that period (it must have existed but it's not well defined in the archaeological record). Balcanic Neolithic had its origins in northern Greece and the rest of European Neolithic cultures is derived from the Balcanic ones, directly or indirectly. The only exceptions could be those of Andalusia (origin unknown but possibly in North Africa) and Dniepr-Don (local evolution influenced through the Caucasus or the Black Sea?).

Overall, taking Greece as startpoint of European Neolithic, most of Europe would be like 40-50% "neolithic" in that plot (at least by PC1), only Basques remaining clearly apart. Italy would be like 80-100% "neolithic" but that actually must reflect succesive waves of Aegean (not just Greek but also Anatolian) immigration.

Like other parts of Mediterranean Europe the Neolithic colonization of the carriers of the Cardium Pottery culture was unequal: some sites were obviously colonized while many others seem just aculturized. Like in Mediterranean Spain or France, or Sardinia for the case, that should not ammount to such high levels of "Eastern genetics", not at all.

But Italy, specially the south, was in the succesive periods after Neolithic heavily influenced by the Aegean: it happened in the Chalcolithic (with partial exception in the Megalithic period maybe), then in the Bronze Age and then in the Iron Age with all those Greek colonies and, of course, the Etruscans that this article deals primarily with. Even in the Roman period, Italy was strongly influenced by Greece and eventually even gave up its prominence to Byzantium, renamed Nea Roma and nicknamed Constantinople.

So I guess that all those waves can explain the somewhat more eastern component of most Italians, not very different from that of Greeks (notice this is somewhat different in the Y-DNA).

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