However, I'm under the impression that the Roman Empire was a lot more linked than we think when we say "they didn't have cars or airplanes".
Sea/river transport has been available since the first monkey climbed on a floating log and it was already widespread, cheap and convenient even 2k years ago.
Human population was only a tiny fraction of 20th century numbers back then. There were no casual travelers from Constantinpole to Buenos Aires at all.
Sea/river transport has been available since the first monkey climbed on a floating log and it was already widespread, cheap and convenient even 2k years ago.