There is a new paper with some potentially interesting information about hypothetical demographic changes in Northern Europe in recent prehistory.
Linea Melchior et al., Genetic Diversity among Ancient Nordic Populations. PLoS ONE 2010. Open access.
Most of the recovered data is from the last 2000 years and only three sequences are from earlier periods (two from Neolithic and one from Early Bronze). This limits somewhat the conclusions that can be reached from this paper alone but will be useful to complement the previous data and gradually draw a clearer picture of ancient genetics in Europe. Still they make some meaningful findings that I comment below.
Demographic replacement with Urnfield culture in Northern Europe?
The finding of haplogroup U4 and U5a among Neolithic Danes and yet another case of mtDNA U4 in the single Bronze Age successful sample, essentially discards the hypothesis of Neolithic replacement so far north (Bramanti 2009). It still remains as a possibility for Central Europe but this hypothesis is largely reliant on Epipaleolithic and Neolithic Era foragers' data from the Baltic area (essentially U5 and U4 as well) and a limited sample from Paleolithic Swabia.
The haplogroup frequencies for antique and historical Danes are very similar to present. What says that, if there was any replacement, this happened after the Early Bronze Age.
In this sense, I am considering more and more the possibility of a demic replacement in Northern Europe with the Urnfield culture expansion, because a similar situation is apparent in the Elbe basin, where the Corded Ware site of Eulau and previous Neolithic samples show mtDNA apportions quite different from modern (high K in Eulau, high N1a in Danubian Neolithic of East Germany, low H in all), while the nearby site of Liechtenstein, belonging to Urnfield culture (Late Bronze Age), already displays a very modern mtDNA pool, high in H and U.
Notice please that while this may apply to Northern Europe, it is certainly not the case in the South, specially in the Southwest, as well as in Morocco, where mtDNA H is found at modern frequencies more or less at all temporal layers since Late Upper Paleolithic. Sometimes partial data for Northern or Central Europe is happily extrapolated to the whole continent and this is very much incorrect.
The vanishing of haplogroup I
This is even more intriguing, specially because this paper findings (12.5% of haplogroup I) are highly consistent with previous data showing high apportions of mtDNA I in Denmark (all for the last two millennia), when now this haplogroup only amounts to 2.5%.
As I say, the rest of the samples for this period are totally modern but this identity is strangely broken when we consider haplogroup I, which has shrunk dramatically and nobody seems to understand why.
The authors notice that haplogroup diversity appears to have been higher in the past than today, which is surely related, but they are not able to propose a cause for this phenomenon other than drift.