It is possible that R1b started in C Asia, went o'er the 'ol Caspian, and then, blocked by the climactic ice, dipped back down into the caucasus and from there into asia minor and onwards to europe. Maybe a few trudged along hugging the northern coast of the Black Sea as well. Yeah I know about C Asia but I am talking realistically here (discounting all herrenvolk fantasies).
By the time that R1a expanded there was no ice block and it completely overwhelmed E Europe.
Later, the Hatti followed the transcaucasus route into Asia Minor, while the Mittani, Sindoi, Kassites, and the Greco-Persians took a more direct ME route, along with their greekoid slaves.
It is also possible that the inland seas do not present any effective challenges to migration.
By the time that R1a expanded there was no ice block and it completely overwhelmed E Europe.
Later, the Hatti followed the transcaucasus route into Asia Minor, while the Mittani, Sindoi, Kassites, and the Greco-Persians took a more direct ME route, along with their greekoid slaves.
It is also possible that the inland seas do not present any effective challenges to migration.