Anthony also suggests that the proto-Indo-European language formed mainly from a base of languages spoken by Eastern European hunter-gathers with influences from languages of northern Caucasus hunter-gatherers, in addition to a possible later influence from the language of the Maikop culture to the south (which is hypothesized to have belonged to the North Caucasian family) in the later neolithic or bronze age involving little genetic impact.
https://en.wikipedia.org/wiki/Proto-Indo-Europeans
https://en.wikipedia.org/wiki/Proto-Indo-Europeans