Friday, 8 February 2013

Etruscan mtDNA origins (Ghirotto et al. 2013)

From the paper:
A model of genealogical continuity across 2,500 years thus proved to best fit the observed data for Volterra, and especially Casentino, but not for another community dwelling in an area also rich with Etruscan archaeological remains (Murlo), nor (as expected) for the bulk of the current Tuscan population, here represented by a forensic sample of the inhabitants of Florence.
As for the second question, the IM analysis shows that indeed there might have been a genealogical link between modern Tuscans and the inhabitants of what Herodotus considered the Etruscans’ homeland, Western Anatolia. However, even under the unrealistic assumption of complete reciprocal isolation for millennia, the likely separation of the Tuscan and Anatolian gene pools must be placed long before the onset of the Etruscan culture, at least in Neolithic times; if isolation was incomplete, the estimated separation must be placed further back in time. Consistent with this view is the observation that Etruscan and Neolithic mtDNAs are close to each other in the two-dimensional plot of Figure S4C; however, a formal test would be necessary to draw firm conclusions from the simple observation of a genetic similarity. Separation times were very close when estimated both using a sample from Western Anatolia, and an expanded sample including individuals from much of Anatolia, and so the choice of the Anatolian population does not seem to affect the results of this analysis.
As always with estimates in years, the choice of mutation rate may affect results, but I am reasonably confident that this particular result does not depend on such issues. From the paper:
For these tests we chose the mutation rate (μ) estimated from the data in the previous ABC analyses (very close to the figure accounting for the time-dependency of the mitochondrial molecular clock [13], μ = 0.003). Tests were also run using the value incorporating a correction for the effects of purifying selection [23] (μ = 0.0014), always finding that it results in a further increase of the estimated separation times (Figure S7B). Only assuming very high mutation rates, at least twice as large as estimated in Henn et al. [13], was it possible to obtain separation times less than 5,000 years (Figure S7B). With both Anatolian samples, any degree of gene flow after separation between the ancestors of Tuscans and Anatolians resulted in more remote separation times.
A couple of observations:

If Etruscans did originate in Anatolia then presumably the historical Etruscans were not descended entirely from them but from a mixture of pre-Etruscans with the incoming population. So, it would seem that the inferred dates are incompatible with a folk migration model of Etruscan origins, but not necessarily with a model that accommodates admixture (e.g., initial mtDNA gene pool separation c. 8,000 years ago with the onset of the Neolithic + later admixture during the Bronze Age). On the other hand, the close similarity between Etruscan and Central European Neolithic mtDNA is a good argument for (mostly) continuity in this case.

That things did happen in Italy in the last 5,000 years can be inferred on the basis of the Iceman's genome. It will certainly be interesting to extract Y chromosomes and/or autosomal DNA from some of these Etruscan samples.

A different issue that may bias dates upwards is the occurrence of East Eurasian mtDNA in current Anatolian Turks. It is not clear by how much this would affect age estimates (this admixture is low, sub-10%, but from a population that split off from West Eurasians perhaps more than 40kya); it would nonetheless be useful to repeat the experiment after either (i) purging the Anatolian sample of lineages likely to have introgressed into the population in medieval times, or (ii) using a different West Asian sample other than that of Anatolian Turks.

In any case, it's great to finally have the genetic characterization of a historical European people, and hopefully more samples will follow both from Italy (at least from those who practiced inhumation) and elsewhere.

PLoS ONE 8(2): e55519. doi:10.1371/journal.pone.0055519

Origins and Evolution of the Etruscans’ mtDNA

Silvia Ghirotto et al.

The Etruscan culture is documented in Central Italy (current Tuscany and Northern Latium, formerly known as Etruria) between the 8th and the 1st century BC. Questions about the Etruscans’ origins and fate have been around for millennia. Herodotus and Livy regarded them as immigrants, respectively from Lydia, i.e. Western Anatolia, or from North of the Alps, whereas for Dionysius of Halicarnassus they were an autochthonous population [1]. Previous DNA studies, far from settling the issue, have raised further questions. The Etruscans’ mitochondrial DNAs (mtDNAs) appear similar, but seldom identical, to those currently observed in Tuscany [2], [3]. Assuming reasonable effects of genetic drift and mutation, these levels of resemblance proved incompatible with the notion that modern Tuscans are descended from Etruscan ancestors [4], [5]. Explanations for this result include the (extreme) possibility that the Etruscans became extinct, but also that their modern descendants are few and geographically dispersed, or that the ancient sample studied represents a small social elite rather than the entire population [4]. As for the Etruscans’ origins, ancient DNA is of little use, because pre-Etruscan dwellers of Central Italy, of the Villanovan culture, cremated their dead [1], and hence their genetic features are unknown. DNAs from modern humans and cattle in Tuscany show affinities with Near Eastern DNAs, which was interpreted as supporting Herodotus’ narrative [2], [6], but in these studies modern Tuscans were assumed to be descended from Etruscan ancestors, in contrast with ancient DNA evidence [5]. The claim that systematic errors in the Etruscan DNA sequences led to flawed genealogical inference [2], [7] is not supported by careful reanalysis of the data [8].


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