A growing human population requires additional food and energy supplies to support it, and given the agricultural technology available in 1400, those increases could come from but three sources: bringing more land under cultivation, increasing the labor inputs on a given plot of land (including selecting better seed), or increasing the amount of water or fertilizer.
This causes some problems as it becomes naturally harder to support more and more people.
Moreover, those densely populated regions of Earth corresponded to just fifteen highly developed civilizations, the most notable being (from east to west) Japan, Korea, China, Indonesia, Indochina, India, Islamic West Asia, Europe (both Mediterranean and West), Aztec, and Inca.
People mainly lived in numbers back then, as it was not wise to live without community and not close to people.
As an environmental historian myself, I found that observation compelling and incorporated an ecological theme in my narrative.
This shows the authors credentials by stating that he is a primarily environmental historian.
Later, during the Enlightenment of the seventeenth and eighteenth centuries, they attributed their superiority to a Greek heritage of secular, rationalistic, and scientific thought.
I find it interesting that they changed their religion as time passed.