the Reddit comment:https://www.reddit.com/r/AskHistorians/comments/33it26/is_globalization_a_new_phenomenon_or_just_a/ >the old cultural cores of Rome, northern India and the Chinese central plains experienced a dramatic decline in urbanization Oxford center for late antiquity: http://www.ocla.ox.ac.uk/sect_lre.shtml
In a recent Comment on this subbreedit , a person said that the cultural cores of Rome declined during the 3rd century but oxford center for late antiquity says the late roman empire began declining in 379. So where did all the people and culture go? Where did the cultural centers[cites}move to? Except the obvious example of the move of Rome to Constantinople?
There a rather nasty round of the Plague during the tenure of Marcus Aurelius (180ish CE) that killed up a 3rd of the population and took a hefty bite out of the Roman Army. Its more than likely that the plague hit the population centers harder than the country. Then starting in 250 till about 550 CE there was a prolonged period of climate instability that disrupted the agricultural underpinnings of the Empire and it's neighbors.
We will probably never know exactly what happened, but we do know that cities until modern times did not have a self sustaining populations. They relied on immigration from the country to maintain their populations. One possibility is that if the hinterland could not produce the consistent surpluses it did in the past, more marginal food productions techniques would be need to be employed, which would slow population growth, and limit migration to cities. Urban population would decline, and the higher price of labor (fewer people) would keep the economic focus on agricultural production.