How likely is a "Pontic" language family linking languages from Northwestern Caucasus with Proto-Indo-European? The Yamnaya people had a lot of Caucasus ancestry, could some tribe from the Caucasus have brought it's language with it and introduced it to the native Eastern European hunter-gatherers? Is there any good evidence for this connection, or has it been completely refuted?
A number of languages: Basque, Kartvelian and Caucasic, have prefixal nominal morphology, especially prefixes a-, ma- and tsa-. None of this exists in PIE. So PIE belongs to another family than Basque, Kartvelian and Caucasic. A family ("Pontic") that would put together Caucasic and PIE does not make sense. Now the issue of crosslinguistic borrowings is another issue.