The data shown in the map comes from a Yale-led study published earlier this month in Scientific Data, which compiled the most comprehensive dataset on historical urban populations to date.

The data has a number of limitations and is “far from comprehensive.” Certain parts of world are better represented than others, and some well known cities do not appear until centuries after they were founded. That said, it is the first global-scale collection of pre-1950 urban populations, and a good starting point for future research.