What is Diamond's main argument in Chapter 5 of Guns, Germs, and Steel?
In Chapter 5, Diamond does not really make much of an argument. He is simply presenting facts without trying to make a point. He is using this chapter to set up other chapters.
In Chapter 5, Diamond is telling us which areas of the world discovered food production on their own and which areas only got it through diffusion and borrowing. To the extent that there is an argument, he is claiming that the places that got a head start on food production came to have an advantage over those that did not. He argues that the places that got food production earlier became the "haves" of history.