looks like the wave of Indo-Iranians had moved along the Oxus (Amu Darya River),
then ran into the Pamir Mountains where it split into the Proto-Iranians and Proto-Dardo-Nuristano-Indo-Aryans.
Also see the the
map of Indo-European migration for additional description.
area with the yellow border indicates the region of present-day maximum Indo-Iranian
language diversity (not necessarily a homeland). Red dot lines show mountain ranges
which serve as migration barriers.
was the area of Proto-Iranian Urheimat probably located along the Oxus? Because
people need water to live, and this is the main water artery in the area.
The ancient Oxus seemed to have changed its course several times because of the
instability of the river bed flowing through the sands. Consequently, the Aral
Sea seems to be a recent formation (most likely after the beginning of the CE),
and presently it is almost gone again.
Why not the Yaxartes (Syr Darya
River)? Because if the main migration route coincided with the Yaxartes, we'd
had a completely different geographical distribution of the Indo-Iranian languages,
for instance, we should expect they would be dislocated more to the east, and
less to the west, hence occupying the area of Kyrgyzstan, but hardly Iran (based
on a logical conclusion from the symmetric wave expansion hypothesis).
Finally, there are some complicated issues concerning the origins of Scythians/Sarmatians.
The current lexicostatistical
comparison of Indo-European languages shows that modern Ossetian
(which includes Iron and Digor) is rather equidistant from any other Iranian languages.
(2004) arrived at the same conclusion, though he suggested some
distant proximity to the Saka(n) languages as well, therefore the split of Proto-Osstian
must have nearly coincided with the differentiation period of Proto-Iranian. Whether
the proto-Scythians used the Oxus or Yaxartes to travel north, is not really clear,
though the latter possiblilty seems more plausible.