The successful stimulus-response approach to the organization of behavior has been the dominating paradigm for much of the psychology and neuroscience of the 20th century. Martin Heisenberg is a pioneer in championing the idea that all brains, even comparatively simple ones such as those of insects, instead operate according to output-input principles. Since the 1970s, his research produces evidence that the fruit fly, Drosophila melanogaster, is capable of spontaneous behavioral activity, and that the flies use it to control sensory input (i.e., operant behavior). Today, more and more evidence is accumulating also from fields outside of neuroscience that, indeed, one of the common, defining principles of all brains is this concept of operant behavior. Drawing from this evidence, it becomes clear that the conceptually simple process of generating activity and evaluating its consequences forms one of the fundamental cornerstones not only for all of our human nature, but also for our social coherence. This review recapitulates Heisenberg’s most critical experiments and provides an overview over the current literature on the role of spontaneous activity in the ecology and evolution of brains. I conclude that spontaneous activity is both a necessary prerequisite and an inevitable consequence of evolution.