The author tries to answer the question of why certain societies flourished over time and conquered other less evolved societies. Part of his theory is that the Eurasian land mass is on a horizontal axis creating vast regions with similar climates. These regions would allow for easier travel, trade, and commerce than the vertical regions of the Americas and Africa that have vastly different climates as you go North and South.

Easier travel, trade and commerce increased the spread of both innovation and disease. So as farming and bronze spread, so did small pox and cholera. Thus the people in Eurasia developed better technology and immunity to germs. They also had the horse and rice, but my takeaway from this is greater interaction amongst different groups of people enhances innovation. The more conversations, the more learning.