Ancient DNA is painting a remarkable picture of the period of European prehistory known as the Late Neolithic/Early Bronze Age. It's showing that after the collapse of genetically Near Eastern-like farming populations of middle Neolithic Central Europe - probably as a result of climate fluctuations, disease, famine and increasing violence - the vacuum was filled by genetically much more European-like groups from the eastern and western peripheries of Neolithic Europe.
First came the settlers from the east, belonging to the vast archeological horizon known as the Corded Ware Culture (CWC). About three hundred years later they were joined in Central Europe by migrants from the Atlantic Fringe, belonging to the Bell Beaker Culture (BBC). During the early Bronze Age, the CWC disappeared, and was replaced by the Unetice Culture (UC), which briefly overlapped with the late BBC.
Ancient DNA recovered to date suggests that the Bell Beakers were genetically the archetypal Western Europeans, characterized by Western European-specific mtDNA H subclades and Y-chromosome haplogroup R1b. Interestingly, R1b has also been found among remains of aboriginals from the Canary Islands, just off the coast of northwest Africa. It might be a stretch to attribute this directly to the Bell Beakers, but they were certainly capable sailors, so perhaps not?
On the other hand, the CWC and UC populations appear to have been Eastern Europeans to the core, carrying relatively low levels of mtDNA H, and showing strong mtDNA affinity to Bronze Age Kurgan groups of Kazakhstan and South Siberia.
Here are a couple of figures from recent studies, Brandt et al. and Brotherton et al., respectively, illustrating much of what I just said.
So it seems everything is falling into place, with ancient DNA, archeology, and modern European genetic substructures all showing basically the same phenomenon.
However, for a while now the ever more precise present-day phylogeography of R1b has been hinting that this haplogroup might not have expanded across Europe from the west. That's because the most basal clades of R1b are found in West Asia, and its SNP diversity decreases sharply from east to west in Europe. Below is a schematic of the latest phylogeography of R1b. It was presented at the recently held 9th Annual International Conference on Genetic Genealogy by Arizona University population geneticist Dr. Michael Hammer.
And here is another map shown by Hammer at the same conference, illustrating the frequencies of various R1b subclades across Europe.
I didn't see the presentation, so I don't know what Hammer actually said. But it appears as if his theory is that R1b spread across Europe from the Balkans during the late Neolithic or later, and then exploded in-situ from certain areas of Central and Western Europe during the metal ages. If true, this scenario obviously doesn't match the presumed west to east expansion of the Bell Beakers.
But here's yet another slide from Hammer's talk, which shows the frequency peaks of the most common European subclades of R1b: U106, L21 and U152. Curiously, these peaks are all located in and around former Bell Beaker territory (second image below, from Wikipedia).
Admittedly, we only have two Y-chromosome results from Bell Beaker remains, both from the same site in Germany dated to around 4500 YBP, but both belonging to R1b. Based on that, plus all of the indirect evidence outlined above, it's already very difficult to shake the association between the Bell Beakers and R1b. So I'm thinking there are three possible explanations why the latest R1b phylogeography doesn't support a Bell Beaker-driven expansion of this haplogroup in Europe.
1) The current mainstream theory positing the origin of the Bell Beaker Culture in Portugal is wrong, and the earliest Bell Beakers expanded from East Central Europe, as was once thought.The first option basically ignores ancient mtDNA data which shows that the Bell Beakers of Central Europe were of Iberian origin, at least in terms of maternal ancestry. So for now, I'm going with the third option, and looking forward to more ancient DNA results.
2) The latest R1b phylogeography is based on limited sampling, and many more individuals need to be tested from former Bell Beaker areas in Iberia and France to catch the basal R1b subclades in these regions.
3) The people who were to become the Bell Beakers in Iberia originally came from the southern Balkans, via maritime routes across the Mediterranean, and then dominated Western and Central Europe via a series of migrations and back migrations. The latest R1b phylogeography is simply not intricate enough to properly describe this complicated process.
A lot can be said about what might have pushed the Balkan proto-Bell Beakers to Western Europe during the late Neolithic, if they actually existed. At the time Bulgaria was being invaded by steppe nomads from just north of the Black Sea, and its agricultural communities were disappearing rapidly. I suppose the ancestors of the Bell Beakers might have been refugees trying to escape these nomads. Then again, perhaps they were the descendants of the nomads who learned to sail after reaching the Mediterranean? I might revisit the issue when I have more data to work with.
Michael Hammer, Origins of R-M269 Diversity in Europe, University of Arizona, FamilyTreeDNA, 9th Annual Conference
Guido Brandt, Wolfgang Haak et al., Ancient DNA Reveals Key Stages in the Formation of Central European Mitochondrial Genetic Diversity, Science 11 October 2013: Vol. 342 no. 6155 pp. 257-261 DOI: 10.1126/science.1241844
Brotherton et al., Neolithic mitochondrial haplogroup H genomes and the genetic origins of Europeans, Nature Communications 4, Article number: 1764, Published 23 April 2013, doi:10.1038/ncomms2656 See also...
Latest speculation about R1b
High mtDNA affinity between Bronze Age Minoans and Western Europeans