But we can determine that there was a population movement from Europe to Asia during the Bronze Age, and judging by this latest study these people carried R1a-Z93.
Sure there was population movement from Europe into Asia. And allot of it is even recorded. Like Scythians, Cimmerians etc. But the question we need to ask ourselves is, Coudln't this have been rather a "back migration" ?
It could be that the Indo_Iranian branch of Indo European languages has a Northern Origin in is a signal of back migration into West and South_central Asia. These could have brought z93 with them.
But What if the ancestory of this Northern Indo European tribes (like the Balto_Slavs, Indo_Iranians etc.) had their root coming from somewhere else, like Western Asia for example. This would make the Indo_Iranian/Balto_Slavic only a second expansion and not initialy the first.
The new foundings that the Kurgan people had darker features to modern Europeans speak for a population movement into this area in my opinion.
There are two reasons for me to think that way. If they were simply local Hunther and Gatherers, they would have alleles for dark skin and light eyes. But the Kurgan people where rather medium skinned and mixed eyed with the majority beeing brown. So it is unlikely that they were local Hunthers and Gatherers. Also the Kurgan culture seems to have roots in Mesopotamia.
There are few possible scenarios in my opinion.
the most likely two in my opinion.
1. Proto_Indo Europeans moved from a region between West and South_Central Asia into the Steppes/Northern Caucasus and from their they spread into Europe. The Indo_Iranian branch could have existed already in Western Asia or they evolved later in the Steppes/Northern Caucasus and from there migrated around the globe.
2. Proto_Indo Europeans ultimately evolved in the Pontic_Caspian steppes and migrated from there around the globe. But the Proto_Proto Indo Europeans came from somewhere else, again West Asia is the best candidate.