It looks like Sredny Stog was the early vector for the spread of both Anatolian Neolithic and Caucasus hunter-gatherer (CHG) admixture onto the steppe, from the west and east, respectively:
These data testify the assumption about the existence of mixed Tripolye-Sredniy Stog marriages, because Tripolye population represented the Mediterranean anthropological type according to the not numerous Tripolye burials (Потехина 1999, c.154). It is interesting, that the massive Protoeuropoid type was typical for the oldest and the most eastern monuments of Sredniy Stog, while mesomorphic Mediterranean type was typical for the Igren cemetery, which was one of the youngest monuments related to the second and third periods of the Sredniy Stog culture and synchronous to the Tripolye B I and B I-II.
Appearance of pottery with pearls at the settlements of the third period of Sredniy Stog culture and glossy ceramics without ornamentation in the eastern variant sites, as well as the group of vessels with the steppe traces at the Svobodnoe settlement, allow me to assume the existence of mixed marriages between the Sredniy Stog and Northern Caucasus population.
Source: Early Eneolithic in the Pontic Steppes, book by Nadezhda Sergeenva Kotova, available at Academia.edu here.