In an ideal homeostatic world, there would be little change. However, in a world such as Earth, dominated by living things, there can be no stasis, no equilibrium. For any organism, the successful continuation of life requires the ability to adjust to changed conditions. The paleoenvironments that were the contexts of past human actions must be known if we are to understand human history and evolution. Social environments, the crucial contexts of human planning and decision-making, are the subjects of anthropological and archaeological social theory, with their own vast literatures. In this chapter attention focuses on human strategic responses to changes in physical and biological environments, distinguished as much as possible from social environments.

Environmental change, loosely defined, is a departure from the “mean” or perceived normal state or condition of any aspect of the environment. Humans respond only to those changes that they perceive, and then only to those that affect conditions or resources that are important to them. To illustrate with a simplified example: a competitive replacement of one species of mouse by another on a mountainside should evoke no response whatever from a community of farmers in valleys nearby. The feeding habits of the new mice, however, could initiate changes in the ratios of grasses available to grazing animals, which might be crucial to pastoralists using the highlands. The pastoralists would observe the vegetative change, and might try to mitigate it by firing the grasses; they might not recognize the subtle role of the mice in the changed conditions.