I updated the story of R1b-V1636. This lineage along side of small number J1-s was almost certainly involved in the introduction of Caucasian genes in to Eneolithic Steppe reaching Volga region. Before Khvalynsk and Sredni Stog the Caucasian shift was absent from Steppe while in EBA the homogenized Caucasian shift became the main signature of Yamna and CWC.
Offcourse the data is still insufficient to make definitive judgements about the PIE homeland but wherever it was it couldn't be very far from Caucasus. At last if we rely on those Steppic migrations.
From linguistic point of view this situation nicely supports the Ranko Matasovic's paper.
Areal Typology of Proto-Indo-European: The Case for Caucasian Connections
This paper re-examines the evidence for early contacts between Proto-Indo-European (PIE) and the languages of the Caucasus. Although we were not able to find certain proofs of lexical borrowing between PIE and North Caucasian, there are a few undeniable areal-typological parallels in phonology and grammar. Some features generally attributed to PIE are not found in the majority of languages of North and Northeastern Eurasia, while they are common, or universally present, in the languages of the Caucasus (especially North Caucasus). Those features include the high consonant-to-vowel ratio, tonal accent, number suppletion in personal pronouns, the presence of gender and the morphological optative and, possibly, the presence of glottalized consonants and ergativity.