Ecological and evolutionary considerations suggest that radiation hormesis is made up of two underlying components. The first (a) is background radiation hormesis based upon the background exposure to which all organisms are subjected throughout evolutionary time. The second and much larger component (b) is stress-derived radiation hormesis arising as a protective mechanism derived from metabolic adaptation to environmental stresses throughout evolutionary time especially from climate-based extremes. Since (b) > > (a), hormesis for ionizing radiation becomes an evolutionary expectation at exposures substantially exceeding background. This biological model renders linear no-threshold theory invalid. Accumulating evidence from experimental organisms ranging from protozoa to rodents, and from demographic studies on humans, is consistent with this interpretation. Although hormesis is not universally accepted, the model presented can be subjected to hypothesis-based empirical investigations in a range of organisms. At this stage, however, two consequences follow from this evolutionary model: (1) hormesis does not connote a value judgement usually expressed as a benefit; and (2) there is an emerging and increasingly convincing case for reviewing and relaxing some recommended radiation protection exposure levels in the low range.