Estimates of the empire’s population at any particular time are educated guesses and vary widely. And the numbers in any particular location also fluctuated over time because of disease, famine, war or for reasons that aren’t always apparent today. Urbanization and related factors may have caused the empire to out-build its resources.

In the first century, Rome was probably the largest city in the world, with a population reaching perhaps 1 million. In later centuries it became a backwater and, at one point, its population dwindled to the size of a small town.

Which came first: farms or cities? It isn’t a chicken-egg question. Isolated families and clans remain hunter-gatherers. The archeology of Neolithic sites indicates that towns and cities made organized agriculture necessary.