First, there's no archaeological evidence that Steppe related cultures of 4-3 millenium BC moved in a westerly direction further than lower Danube area. Hypothesis that all of the single grave traditions in Europe were of Steppe origin is debunked many years ago, since 1960's. Than, North-Pontic region itself was object of invasions for all time, even so-called genetic "Steppe admixture" was brought there from south Caucasus, also there're invasions of Maykop, Neolithic and Corded cultures into steppe, so IE language appeared there exactly with some of them, not otherwise. Also, North Pontic populations were of very specific mix of Mesolithic ans Neolithic lines like these ones,
and unparalleled in the late true IE populations
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