Berun, you forgot about Khvalynsk culture (Chalcolithic Samara), where there was both R1a and R1b.
I0433 / SVP46 (grave 1) - R1a1
I0122 / SVP35 (grave 12) - R1b1
They were from this cemetery:
"The cemetery of Khvalynsk II, Saratov oblast, Russia, on the west bank of the Volga River,
6 km north of the village of Alekseevka. Khvalynsk I and II are two parts of the same cemetery,
excavated in 1977-79 (Khvalynsk I) and 1987-88 (Khvalynsk II).23 The two excavations
revealed 197 graves, about 10x larger than other cemeteries of this period in the Volga-Ural
steppes, dated by radiocarbon to
5200-4000 BCE (95.4% confidence). Bones of domesticated
cattle and sheep-goat, and horses of uncertain status, were included in 28 human graves and
in 10 sacrificial deposits. The 367 copper artifacts in the graves, mostly beads and rings, are
the oldest copper objects in the Volga-Ural steppes, and trace elements and manufacturing
methods in a few objects suggest trade with southeastern Europe."
This shows that ca. 7000 years ago there was at least some overlap between R1a and R1b.
We know that R1a people lived from Karelia (sample
I0061) to Khvalynsk (sample
I0433).
We don't know how far south did Non-V88 R1b extend, it could by more southerly than R1a:
http://s32.postimg.org/npur1zskl/R1b_R1a.png
