Genetics is now being brought to bear on whether there were non-trivial population movements in the prehistorical period. Or more precisely, a combination of genetics and archaeology, whereby the archaeologists retrieve and extract genetic material which the geneticists amplify and analyze. This has helped establish that European hunter-gatherers were not lactase persistent. This is totally unsurprising, but was a nice proof of principle. When it comes to ascertaining genetic relationships among populations, as opposed to specific traits whose genetic architecture is well established, it's a bit trickier. Who knows how many population movements may have interposed themselves between the present and a particular period in the past from which you have samples? A new paper in PLoS ONE reports findings which do little to clarify, though add weight to skepticism as to the definitiveness of earlier results, Genetic Diversity among Ancient Nordic Populations:
Using established criteria for work with fossil DNA we have analysed mitochondrial DNA from 92 individuals from 18 locations in Denmark ranging in time from the Mesolithic to the Medieval Age. Unequivocal assignment of mtDNA haplotypes was possible for 56 of the ancient individuals; however, the success rate varied substantially between sites; the highest rates were obtained with untouched, freshly excavated material, whereas heavy handling, archeological preservation and storage for many years influenced the ability to obtain authentic endogenic DNA. While the nucleotide diversity at two locations was similar to that among extant Danes, the diversity at four sites was considerably higher. This supports previous observations for ancient Britons. The overall occurrence of haplogroups did not deviate from extant Scandinavians, however, haplogroup I was significantly more frequent among the ancient Danes (average 13%) than among extant Danes and Scandinavians (~2.5%) as well as among other ancient population samples reported. Haplogroup I could therefore have been an ancient Southern Scandinavian type “diluted” by later immigration events. Interestingly, the two Neolithic samples (4,200 YBP, Bell Beaker culture) that were typed were haplogroup U4 and U5a, respectively, and the single Bronze Age sample (3,300–3,500 YBP) was haplogroup U4. These two haplogroups have been associated with the Mesolithic populations of Central and Northern Europe. Therefore, at least for Southern Scandinavia, our findings do not support a possible replacement of a haplogroup U dominated hunter-gatherer population by a more haplogroup diverse Neolithic Culture.
Here's a review of an earlier paper on this topic. Here's an important section from the discussion of the current paper:
...Given our small sample sizes from these crucial time periods further studies are certainly required. However, the frequency of Hg U4 and U5 declines significantly among our more recent Iron Age and Viking Age Danish population samples to the level observed among the extant Danish population. Our study therefore would point to the Early Iron Age and not the Neolithic Funnel Beaker Culture as suggested by Malmström et al. (2009)...as the time period when the mtDNA haplogroup frequency pattern, which is characteristic to the presently living population of Southern Scandinavia, emerged and remained by and large unaltered by the subsequent effects of genetic drift. In contrast to Hg U4, which is only found in the Neolithic and Early Bronze Age samples, Hg U5 was observed in ~9% (5/53) of the remaining ancient samples and identified at all sites except Kongemarken and Skovgaarde.
I wouldn't put too much stock in these specific results. The sample sizes and representativeness issues are probably such that each new paper is going to change our assessment. But, I think the section which I emphasized points to a shift in the Zeitgeist. Until recently there's been a very strong bias among historical geneticists to assume that the genetic variation is more strongly affected by deep time events, and that recent replacements and perturbations will have less impact. I think there were good reasons for this assumption, and still are, generalizing from broader patterns. But the over-extrapolation of the rule-of-thumb may have led to models which will soon be falsified in many specific instances. On a slightly bittersweet note, ancient DNA will be able to answer questions about the origins of many circumpolar populations, but will have far less to tell us about societies and cultures further south, simply because of less favorable conditions for preservation. The main exception to this truism will presumably be desert societies. For example, Tutankhamun has been typed as of the R1b Y lineage.