Central Eurasia is a linguistic bottleneck, spread zone, and extinction chamber, but its languages had to come from somewhere. The locus of the IE sp… - Johanna Nichols

Approximately every two millennia, then, there has occurred a spread of a language family from a locus in the eastern part of the central Eurasian spread zone to cover the steppe and central Asia, extending partially or intermittently to the Danube plain, Anatolia, and northern Mesopotamia. The loci of the historically attested spreads are near the edge of the spread zone rather than in the centre of it: the piedmont to the south (Bactria-Sogdiana) for Iranian, the north of Mongolia for Turkic and Mongolian. The trajectories of language spread run east to west along the steppe and through the desert to the Near East as shown .... To take clear and historically well-attested examples, the locus, trajectories, and range of IE must have been much like those of Iranian or Turkic. ... The placement of the locus specifically in the vicinity of Bactria-Sogdiana is justified in .... A homeland reconstructed as locus, trajectory and range removes the dilemma: a locus in the vicinity of Bactria-Sogdiana implies a spread beginning at the frontier of ancient Near Eastern civilization and a range throughout the steppe and central Asia, following the east-to-west trajectory, with occasional or periodic spreads into the Danube plain and Anatolia.

The Iranian family, which was next to sweep across the steppe and deserts, finds its region of greatest diversity in the central Asian mountains, and its ancestral Indo-Iranian family finds its own greatest diversity in the mountain region from central Asia to northern India (i.e. Bactria- Sogdiana and parts just south).

There are several linguistically plausible possibilities for the origin of Pre-PIE. It could have spread eastward from the Black Sea steppe (as proposed by Mallory 1989 and by Anthony 1991, 1995), so that the locus formed only after this spread but still very early in the history of disintegrating PIE. This large eastward spread would be a departure from the general westward trajectory of spreads in central Eurasia, occasioned by the epoch-making domestication of the horse and development of wheeled transport. Pre-PIE could have been a central Asian language long before its rapid spread; if so it had a large range before its expansion, and the dispersal began with the development of the locus in a pre-existent range. It could have come into the spread zone from the east as Mongolian, Turkic, and probably Indo-Iranian did.