Marks considers the environmental and economic state of the world by the year 1400. Today, the human population exceeds eight billion, but in 1400 it was only 380 million. Almost all people lived in rural areas, and human settlements covered 7% of the land on Earth. Although the world has many more people today, 70% of them live on the same 7% of land or 4.25 million square miles as their ancestors in 1400. In addition, Marks identifies 15 advanced civilizations. These civilizations were the ultimate product of an “agricultural revolution” that began in modern-day Iraq when people transitioned from hunting-and-gathering to permanent agricultural settlements. The settlements gave rise to social hierarchies, in which most of the population made food through agriculture and an elite of priests and rulers needed to explain and protect society. The settlements also gave rise to artisans, who made and maintained the items necessary for society and culture, and to trade between cities and tribes.

Even by 1400, the world’s largest urban populations amounted to only 1% of the global population. Nine of the world’s largest cities were in China, and the biggest was Nanjing. The second largest city was Vijayanagar in southern India, and the third was Cairo in Egypt.