We build on the model of Chapter 3 to explain how sedentism could have developed in response to better climate conditions involving higher means and lower variances for temperature and rainfall. Sedentism is defined to mean a willingness of human populations to stay at the same site for multiple generations despite occasional periods of low productivity in relation to other sites. We identify three causal channels leading to sedentism. First, there is a short-run channel where climate improvement leads agents to remain at sites when weather there is temporarily bad, because when conditions are harsh, they are less harsh than they were under the previous climate regime. Second, there is a long-run channel where better climate leads to higher regional population. This causes some people to remain at sites where weather is temporarily bad because sites with good weather are now more heavily occupied than before. Finally, there is a very-long-run channel where higher regional population leads to the use of previously latent resources and technological innovation. These mechanisms help to explain the rise of large sedentary communities in southwest Asia during the Epi-Paleolithic and in Japan during the early Holocene.

Sedentary foraging is not identical to agriculture, which involves cultivation of plants and eventually their domestication. Cultivation appears to have developed in southwest Asia during a large negative climate shock called the Younger Dryas. After a prolonged period of warm and wet conditions during which regional population reached a high level, an abrupt reversion to colder and drier conditions forced this large regional population into a few high quality refuge sites where surface water was available from rivers, lakes, marshes, and springs. The resulting spike in local populations at these sites drove down the marginal product of labor in foraging and triggered reallocation of some labor toward cultivation. Once some populations adopted cultivation, learning by doing reinforced the incentive to engage in it. Eventually climate improved in the Holocene, regional population grew, and agriculture spread. We believe this mechanism accounts for the archaeological facts in the case of southwest Asia, and similar mechanisms might account for other pristine agricultural transitions (e.g., in China and sub-Saharan Africa). Our model clarifies the causal roles of climate, geography, technology, population, and migration in the development of pristine agriculture. It also helps explain why certain regions did not experience pristine agricultural transitions.