According to the recent article on Y-STR variation in Europe, a particular haplotype, 16-13-17-25-10-11-13 (over DYS19, DYS389I, DYS389II, DYS390, DYS391, DYS392 and DYS393) seems to be ancestral for haplogroup R1a and occurs (together with its one-step neighbors) in a frequency of 30.6% in "Eastern Europe" vs. only 0.6% in "Western Europe". This haplotype is highly suggestive of Eastern European origin.
A search of this haplotype in YHRD gave the following result:
Note that red color indicates presence of the haplotype, while blue indicates its absence. It appears that there is core Eastern European area where the haplotype is present, and this includes most (all?) Slavo-Baltic countries. In Scandinavia, the Netherlands, Belgium and the British Isles the haplotype is mostly lacking(*), which seems to indicate its likely absence in the Proto-Germanic gene pool; this is not true for Germany, Austria and Switzerland, reflecting historical contacts with Slavic and Baltic peoples. The haplotype is also mostly lacking in the Italo-Celtic world, and in the Southern Balkans, again illustrating its essentially Balto-Slavic likely origin (in most cases).
The omnipresence of the Eastern European haplotype in Balto-Slavic lands make it likely that it was part of the Proto-Balto-Slavs, and its geographical extent on the map is a rough approximation for the genetic influence of the representatives of that branch of the Indo-European family.(**)
(*) Note that two of the sites of the British Isles are of Indo-Pakistani populations where R1a achieves a high frequency and a third one is cosmopolitan London.
(**) Or possible of Indo-Iranians as the presence of the marker in the Indian subcontinent, as well as in Iran demonstrates. This influence was, however, probably minimal in Europe until recently.