@Holderlin+@Dov
Very interesting posts, trying to put some light into this intricated successions of cultures.
Languages comparisons could put languages close or far according to lexicon as opposed to grammar, both as as opposed to phonetic trends.
I lack knowledge about the ties between agricultural and pastoral vocabularies of diverse linguistic groups; a deep study about this could help here, I think. A possibility could be that a language spoken in Steppes gained strength about the 5000/4000 BC and borrowed agricultural lexicon from Tripolye influenced cultures of West Steppes; what would not exclude some other loans from other cultures (South Caucasus? or else?);
A good "fusion" in Steppes could have produced a partly new language rather homogenous NOW (after loans) before radiation/diffusion under the form of an homogenous enough "PIE", creating the impression of a cool history without any tribulation in the language? At those times these loanwords borrowed through the same filter on a short enough period would show provisory homogeneity and after that would undergo the same phonetic evolutionS than "genuine" PIE words among the "daughters" languages.
Maybe sometime archeology and genetics will find an agreement? Without more data I guess that a lot of the agricultural vocabulary came from Tripolye culture; Catacombs seemed more agricultural than Yamnaya and physically more akin to East-Central Europe, at least the western Catacombs. But where came Catacombs from? Maybe a melting pot? In fact its physical heterogeneity between subgroups (same for mt DNA between West and East?) and the case of western types and DNA in later eastern Steppes cultures point towards a brewing West/East-East/West spanning a long enough time, I think. But if what I say is sensible (loanwords in a short enough time) we can consider that the agricultural package was obtained from West for the most, soon enough, and transmitted only after to East, where by the way, stoke breeding seemed stronger than plants culture. The 'satem' trend, as said by forumers here and there, could have been born early enough, perhaps as soon as CWC, during some language transmission to unkown groups of North-East or East.
&:I was said that the PIE agricultural lexicon was not so developped as believed at first. In accord with what Dov wrote.
&&: this doesn't say us too precisely from where came the pre-agricultural PIE.