In part 2 of Guns, Germs, and Steel, Diamond argues that superior food production was the root cause beneath the ability of Eurasia’s people to develop the guns, germs, and steel that conquered the rest of the world. He promises to discuss in a future section the particular ways food production techniques had this effect around the world, but first he devotes chapter 4 to explaining why food production had this effect at all.

First, a population that can produce more food can also produce more people. Of the plant and animal matter our planet produces naturally, the vast majority is inedible, poisonous, or too inefficient for humans to bother eating. When people control what the land produces, they can choose to raise the plants and animals that are the best available sources of food. Some farm animals produce fertilizer, do farm work, and provide fuel for fires in addition to providing meat and milk. Consequently, herding and farming societies can usually feed ten to 100 times the number of people hunter-gatherer societies can feed.

In addition to these advantages, farming replaces a nomadic lifestyle with a sedentary one. This allows farming cultures to bear and raise more children and store and use food surpluses. Farming societies are in a better position to support full-time leaders, shamans, artisans, and scribes.

After human societies domesticated large mammals, they quickly developed the ability to transport people and trade goods over longer distances. In Eurasia, horses became deadly tools of warfare by 4000 BCE. Later, when people invented stirrups and saddles, they became even more effective.

In Eurasian farming societies, domesticated mammals lived in close proximity to humans and gave rise to most of the infectious diseases that devastated the populations visited by Europeans in the 1500s and later. Smallpox, measles, and flu all evolved from similar diseases in farm animals. The people who domesticated the animals quickly evolved at least partial resistance to these diseases. However, the same diseases wreaked havoc every time they were introduced to populations that had not previously been exposed to them.

Near the end of the chapter, Diamond pauses to note some possible objections to the points he has made so far. He admits that, over the course of history, a few hunter-gatherer societies have developed sedentary lifestyles, specialist workers, and the like. However, he says that only agricultural societies have ever achieved the population density and number of specialists necessary to develop widespread, politically centralized cultures. Moreover, only food-producing societies have achieved significant technological advancement. Diamond concludes, therefore, that a culture’s ability to advance is ultimately dependent on food production and on the availability of domesticable plants and animals.