The population was a lot less, Bill. But that was not a point that I was responding to.
Much of Asia and north Africa was certainly not isolated from Europe. The other regions you mention were. I have no idea why you bring most of these places into this. especially Russia,
By 300-400 AD the Roman empire was split, with the Greeks influencing the eastern part. The Roman legions did recruit increasing numbers of Germanic merecenaries, at least until they turned on their commanders and assisted with the defeat of Rome itself.
What is your point? Really.