There are several linguistically plausible possibilities for the origin of Pre-PIE. It could have spread eastward from the Black Sea steppe (as propo… - Johanna Nichols

Prior to the Turkic expansion, at the beginning of the Iron Age, Iranian spread from somewhere in the vicinity of Bactria, Sogdiana, and the eastern steppe to cover most or all of western central Asia and the entire steppe, much of the Near East at least to eastern Anatolia, and, at least intermittently, the Danube plain, where Slavic vocabulary and ethnonyms attest to a major Iranianization at about the fifth century AD, and where there is good archaeological evidence of a Scythian presence in the mid-first millennium BC...

The archaeological sources on which this summary is based... most often describe the westward cultures as derived from, or extensions from, the eastern ones. Mallory (1989) and Anthony (1991, 1995) interpret the directionality of cultural derivation as west to east. It is the east-to-west directionality of cultural derivation that would be consistent with the east-to-west linguistic trajectory, since spread of a whole culture is likely to involve language spread (and vice versa). A predominantly east-to-west directionality of cultural derivation and descent for the Eneolithic steppe would be a strong indication that the spread zone with its westward trajectories had taken shape. This is a purely archaeological question, but it is important to dating the rise of the linguistic spread zone.

Central Eurasia is a linguistic bottleneck, spread zone, and extinction chamber, but its languages had to come from somewhere. The locus of the IE spread is a theoretical point representing a linguistic epicentre, not a literal place of ethnic or linguistic origin, so the ultimate origin of PIE need not be in the same place as the locus.