This paper has been all over the press and various blogs this week, so I won't spend too much time on it. But I will say that conclusions based on mtDNA need to be made very carefully, especially if there's no other genetic data available to corroborate them. The vast majority of the hunter-gatherers sampled here carried mtDNA U lineages (mostly U4 and U5), while the farmers from earlier studies showed a much greater variety of haplogroups, including quite a bit of N1a. So far, neither population seems particularly close to modern Europeans.
We compare new mitochondrial DNA (mtDNA) sequences from late European hunter-gatherer skeletons with those from early farmers, and from modern Europeans. We find large genetic differences between all three groups that cannot be explained by population continuity alone. Most (82%) of the ancient hunter-gatherers share mtDNA types that are relatively rare in Central Europeans today. Together, these analyses provide persuasive evidence that the first farmers were not the descendants of local hunter-gatherers but immigrated into Central Europe at the onset of the Neolithic.
B. Bramanti et al., Genetic Discontinuity Between Local Hunter-Gatherers and Central Europe’s First Farmers, Published Online September 3, 2009, Science DOI: 10.1126/science.1176869