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.
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.
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.
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.