During the fifth and much of the fourth millennia BC, fully developed Neolithic communities flourished in most of China. Settlements increased dramatically in number, and spread to ever broader and more varied geographic regions, suggesting a rapid growth in population. These developments coincided with a period of warm and wet climatic conditions known as the Mid-Holocene Climatic Optimum, during which some large-scale centennial warming was accompanied by increased precipitation, due to the expansion of monsoon circulation. The deviation of annual mean temperature from today's is estimated at 1°C in South China, 2°C in the Yangzi Valley, and 3°C in North and Northeast China (Shi, Y. et al. 1993). It appears that the northwest of China experienced the strongest warming, as the mean temperature was 3–4°C higher than today near the Tenggeli (Tengger) Desert around 7290–6380 cal. BP (Zhang, H. C. et al. 2000). The duration and amplitude of the Optimum period, as well as the times of its start and end, differed in different parts of China, however, and a cooling/drying event that followed the Optimum was also asynchronous (An, C.-B. et al. 2006; An, Z. et al. 2000; He, Y. et al., 2004). A number of studies show that this episode of climatic deterioration seems to have occurred around 5000 cal. BP in many parts of north China (An, C.-B. et al. 2006; Li, Xiaoqiang et al. 2003; Schettler et al. 2006). These conditions are the environmental backdrop underlying the economic and social decisions made by the Neolithic communities.