Apparently the theory is this; The I2a1b-L621
subclade seems to correlate with the Cucuteni-Trypillian culture.
https://upload.wikimedia.org/wikiped...utline_Map.png
These people were "Indo-Europeanized" like most European cultures dating from that period. Most admins from ftdna will not say that a certain
subclade belongs to a certain meta-ethnicity but to a culture. Maybe due to PCness or whatever although one can construct other explanations, involving later migrations, etc. such as the Slavic migrations to the Balkans for this specific subclade. For example, when I did the Big Y I was told by ftdna admins that my subclade belonged to the Jasdorf culture, not a specific meta-ethnicity. Constructing other explanations regarding meta-ethnicities etc happens more on forums like this one or anthrogenica etc.