This isn’t the only possible explanation. If you compare Y-chromosome haplogroups to mitochondrial haplogroups there are similar cases within even more recent h

This isn’t the only possible explanation. If you compare Y-chromosome haplogroups to mitochondrial haplogroups there are similar cases within even more recent human populations. For instance, European populations almost exclusively come from R1a and R1b Y-chromosome haplogroups that probably originated with the Proto-Indo-Europeans of the Pontic Steppe. However, there are still European mitochondrial haplogroups corresponding to much earlier European populations. In other words, the carriers of the R1a and R1b haplogroups specifically replaced the male population of prehistoric Europe. These were all modern humans so there’s no reason they wouldn’t be able to have viable offspring the other way around. That’s just not how the Proto-Indo-European migrations seemed to work out. That’s not to say that the same thing happened with the Neanderthals. We can piece together the story of the PIE’s from archeology, linguistics, and population genetics to a much fuller degree than the story of the Neanderthals. But it does go to show that you can have significant interbreeding events between human populations that do end up happening exclusively one way around.