The present distribution and abundance of the ursids is but an ephemeral reflection of an evolutionary path that began with the first identifiable bear, the dawn bear (Ursavus elmensis), 20 million years ago in the early Miocene epoch. Although the dawn bear was only the size of a fox terrier, by the Pleistocene its descendents had evolved into some of the largest terrestrial carnivores the world has known. Most bear species evolved in the northern hemisphere although some dispersed and reached South America, Africa and Southeast Asia. Each species had to cope with ecological changes that affected interspecific competition or the availability of food. Apparently the black bear was sufficiently adapted to have survived largely unchanged from what it was like a million years ago. Numerous species went extinct, leaving only the 8 still present today. Some understanding of the evolutionary pressures that the modern bears have evolved through may help us to understand their behavioral ecology. During the Pleistocene, bears at higher latitudes grew large and ecologically plastic while those closer to the equator remained small and became ecological specialists, as predicted by Geist's (1987) dispersal theory. Adaptations of the teeth of ancestral bear species allowed them to be both herbivores and carnivores. This allowed them to develop large size and broad ecological plasticity. Large body size enabled bears to conserve heat, capture large prey, defend carrion, travel great distances, and, as vegetation increased in the diet, to survive on qualitatively poorer food. Quantity and quality of available food and the degree of sexual dimorphism influenced the size of the home range and the evolution of social behavior in each species. Bears show a great deal of individual variation in behavior and may exploit different subniches as a result of learned behavior. Slight differences in phenotype may also influence exploitation of subniches. Recent literature indicates that some terrestrial bear species are more active predators than previously thought and some evidence suggests a degree of scaling between the size of bears and the size of their prey. Social signalling appears to have been influenced by life in forest habitats but is not well understood. We give a preliminary interpretation of the social organization of the present day bears through the interactive framework of proximate ecological pressures, phylogenetic history, and learning. There are likely few populations of bears anywhere in the world whose behavior has not been significantly influenced by man. This may confound our understanding of their behavior and ecology. Remaining populations of bears may not be able to adapt successfully to the combined effects of human predation, disappearing habitat, and climatic change.