Potemkin wrote:The Parni tribe (who later became the Parthians) seem to have originated in either southern Russia or Bactria, and were probably members of that vaguely defined group we call 'Scythians'. As @fuser said, they were about as 'Persian' as the Holy Roman Empire was 'Roman'. Lol.
The Parthians come from the areas between the Eastern shores of the Caspian Sea and Bactria/Oxus River, a region which was in Classical times heavily populated by other Indo-Iranian tribes and a region which was for much of history directly connected with the societies of the Iranian plateau. The Parthians were not Persian, but they were linguistically and culturally related to the Persians, the Medes and other Indo-Iranians, and had a common heritage as Aryans. What contrasted the Parthians is that they retained elements of Central Asian pastoralist lifestyle. This confusion of either conflating the Parthians as Persian or excessively alienating the Parthians from the Persians stems from the common habit of using "Persian" as a reference point (Persian empire, Persian culture, Persian language), while Persians were only a tribe of a larger group. Hence, Oxy confusing Parthians for Persians is more akin to confusing Alamanni and Franks or Angles, Saxons and Jutes (tribes which share a common heritage and culture to such extent that they can be considered a continuum or part of a larger group) rather than the Holy Roman Empire and the Roman Empire (two distinct polities in habited by distinct peoples in different ages).
As for Southern Russia/Eurasian steppes, some of the Indo-Iranian tribes moved there in classical and early medieval ages, but it is also the likely homeland of the Indo-Iranians in more ancient times. The Scythians are a potential candidate of Indo-Iranians who remained there (but it could also be that they returned at a later age, it's unclear). But we do know that at various stages the Scythians were more a confederation of tribes from different ethnicities. Their lifestyle and composition distinguished them from the Indo-Iranian tribes of the Iranian plateau; they have commonalities with the Parthians due to geographic proximity and lifestyle similarity. We don't know whether the ancestors of the Parthians were Indo-Iranians who traveled around Central Asia and Northwestern parts of the Indian subcontinent with the other Indo-Iranians of the plateau, to end up at the shores of the Caspian Sea and eventually take over the Iranian Plateau in a later arrival; or whether they are remnant Indo-Iranians of the Eurasian steppes who took a more direct path along the Caspian shores towards the Iranian Plateau. But even if it is the latter, linguistically their language was closer to that of the Indo-Iranians in the plateau and their culture converged/aligned with that in the plateau.