A lot of fur has been flying in the PIE debate recently which has been brought about by the addition of the most recent Near East genomes. Enjoy the wonder while you can.
I'm now going to take a big bear claw to the notion that PIE can exist without EHG. A number of the chief Harvard scientists are essentially suggesting this possibility, Lararidis, 2018. As a consequence, LPIE becomes a chapter in a kaleidoscope narrative. I'm all about some Gordon Whitakker Proto-Euphratean, but that's a separate topic. This is about PIE, exterior day 1.
I'll offer a short narrative below to mete out where I think the stronger points can be found to dispel a southern, one half of itself origin. Maybe I'll expand on these later.
"Strange Wives Part 1"
1. Proto-Indo-European was the inevitable development of a world-system seeking to gain access to patrilocal, riverine communities in its periphery. Traders cemented their interests with these fisher-woodsmen around the northern half and western inlets of the two seas the old fashioned way - by making them cousins. Look no further than academia or entertainment to see what factors drive marriage decisions (and to see how quickly they dissolve when nepotic incentives dry up). For incipient IE, this had to have happened at leadership levels and perhaps on a more massive scale involving women.
Since these rather small riverine villages were strongly patrilocal, the issues of trade - protection, passage and privilege - could be cultivated by the marriage of a southern-speaking daughter to the foreign lord, with loyalties and shared interests being nursed and expanded generation after generation. There was an incentive for southern outfits in this global economy to develop these relationships, if for no other reason than to crowd out competing interests.
Make no mistake, the contribution of southern ancestry in PIE (the steppe) originates from this sophisticated world-system on a continuous basis, not some well-positioned archaeological culture in the northern reaches. These northern and southern ancestries combined to create PIE.
The development of PIE at its most fundamental level requires interaction between these two spheres. In all likelihood, a global trade language emanating from the Southern Caucasus/Northern Euphrates was widely understood in the basins of both seas, its river villages, and - apparently in many northern homes. Perhaps a dynamic was in place for hundreds and hundreds of years in which very fertile crescenters overwhelmed the men of renown.
"Strange Wives Part 2"
2. What is Late Proto-Indo-European? What should it be?
LPIE is the Corded Ware-ization of the surviving IE groups, excluding Anatolian, Tocharian and Euphratean (if that exists, which I think it does). That means all surviving IE cultures, to which I would include the Bell Beakers, have significant CW ancestry by different means. In the case of the Beakers, it is strangely female-mediated over a short segment of time.
Am I wrong about those points?
One point worth considering is the similar but different situation at transitioning Lepenski Vir. A similar bridal arrangement tamed these patrilocal barbarians, except the stream of ladies is coming from Western Anatolia it seems. You can see the social process is similar, and necessary. What this shows is that the situation was the rule, not the exception.
It's exceedingly difficult to see IE developing south of the seas. The social and linguistic ingredients just aren't there. Well, half of it is.