In Chapter 8 of Guns, Germs, and Steel, Jared Diamond is trying to explain why farming arose in some areas of the world and not in others. He says that there are many areas of the world that are good for farming today but which were never farmed in prehistoric times. He wants to know why people in those areas did not develop farming while people in other areas did. His main thesis in this chapter is that people began to farm in some areas because those areas both good climates and a large number of species that could be domesticated. In other parts of the world, there were not as many species that could be domesticated and so people did not start to farm. This ties in well with the main argument of the book as a whole, which is that environmental factors, and not racial or cultural factors, allowed some areas of the world to become rich and powerful while others did not.
On piece of evidence that Diamond gives in this chapter can be found in Table 8.1. There, Diamond shows us how many species of large-seeded grass existed in various parts of the world. Grasses with large seeds were important for the development of farming because they could be bred into grains that could be cultivated relatively easily. They were much more useful to early farmers than grasses with tiny seeds. In Table 8.1, we see that the majority of all large-seeded grasses in the entire world were found in the Mediterranean zone. This is a piece of evidence that proves that farming originated there because of the availability of species that could be domesticated.
Another piece of evidence is shown on p.141. There, Diamond tells us that there were no wild mammal species that could be domesticated in California, Chile, southwestern Australia, or South Africa. This is much different from the Fertile Crescent, which had four major species (goat, sheep, pig, and cow) that could be domesticated. Therefore, farming arose in the Fertile Crescent and not in the areas without large domesticable animals.
The main point of this chapter, then, is that farming arose in areas where there were many species that could be domesticated, not in areas that had superior types of people.