The book presents an argument that societies evolved the way they did not because of racial superiority, which is the mainstream theory, but because of the geographic conditions that presented opportunities to different societies, these opportunities have more to do with geographic conditions that existed in the world, such as land that was good for growing crops, animals in the region, the ability of a society to sustain itself.
"Eurasian peoples happened to inherit many more species of domesticable large wild mammalian herbivores than did peoples of the other continents." (Diamond)
For example, Eurasian people became farmers, leaving the hunter gatherer lifestyle behind because the land that they inhabited was more favorable to domesticating the wild crops that grew there. As well as the animals that were available for domestication.
"Hence the availability of domestic plants and animals ultimately explains why empires, literacy, and steel weapons developed earlier in Eurasia and later, or not at all, on other continents." (Diamond)
Diamond's theory extends to the availability or access that societies had to information and the exposure they had to other cultures. Groups of people who were exposed to other cultures, inventions, advancements, were in a better position to advance as opposed to societies that were isolated by geographic barriers.
"Certain favorable aspects of local geography allow a group of people who live in that area to more easily produce food. As these people learn to farm the land and raise specific crops rather than forage for them, they find they have more time for other activities. They create rules that benefit their existence, designate certain members to focus on tasks beyond those of pure survival, and thus set up complex societies."