Diamond argues that the "neolithic transition" from food gathering to agriculture evolved gradually, with long periods in which the two modes of food provision coexisted. Several different factors contributed to the shift.

One of the main factors was a lack of wild animals suitable for hunting and a lack of plants suitable for gathering. This in turn was due to either climate change or to animal population declines because of unsustainable volumes of hunting and gathering. This would make agriculture and domestication of animals more attractive.

Next, once one society began to domesticate plants and animals, neighboring societies would emulate them, meaning that the idea would spread rapidly. As agriculture and the domestication of animals is far more efficient than hunting and gathering and allows for urbanization and specialization of labor, societies that had undergone the neolithic transition would have been more powerful and numerous than hunter-gatherer neighbors and would have been able to dominate, assimilate, or exterminate them. The population growth and the increased population density of settled agricultural societies would preclude a return to hunting and gathering due to the need to support a large population. 

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.