<p>Only humans live in continuously and cumulatively changing worlds of their own making—cultural worlds. Theories of cumulative cultural evolution are multidisciplinary, depending heavily on the home disciplines of theorists. Virtually all cultural evolutionary theorists view behavior as fundamental to cultural evolution, but most theories focus on changes in social relations, or human cognition, or human brains. Here we attempt a first approximation to a behavior-based theory that explains the origins and evolution of the human-built world. We include topics such as the origin and evolution of cultural organization and resulting cultural systems, which have not often been addressed in this literature.</p>