The answer to this question can be found in Chapter 6 of Guns, Germs, and Steel. Diamond’s discussion of this specific topic begins on p. 110. Let us look at the five factors that he says helped push people from hunting and gathering to food production.
Factor 1: “decline in the availability of wild foods.” Farming would be more beneficial in areas with less wild food available. Therefore, if the availability of wild foods declined, the benefits of farming would increase and people would be more likely to farm.
Factor 2: “increased availability of domesticable wild plants.” Diamond says that climate changes in some areas such as the Fertile Crescent helped domesticable wild plants thrive. If these plants were more abundant, it would be easier to domesticate them and, thereby, start farming.
Factor 3: cumulative development of technologies for food production. Farming requires a lot of technology. To farm well you need things like tools to cultivate the soil, tools to harvest the plants, and places to store your excess crops. As time goes by, more of these things are invented. As more technology is invented, it becomes easier to farm.
Factor 4: “two-way link between the rise in human population density and the rise in food production.” As food production started, economies were able to support more people living in a given area (rise in population density). But when people settled down in dense villages and towns, food production became more necessary. The two processes fed off one another. Thus, the beginning of food production actually causes societies to need to farm even more to keep up with their growing populations.
Factor 5: farmers’ ability to conquer hunter-gatherers. Wherever groups of farmers and hunter-gatherers came into competition for resources, the farmers eventually won. They had larger population densities and could therefore overwhelm the less numerous hunter-gatherers. This meant that farming populations grew while hunter-gatherer populations shrank.
Diamond says that experts agree that all of these factors were important. The only disagreements between experts have to do, he says, with the relative importance of the various factors.