We propose that the kelps (Laminariales) radiated in the North Pacific following the onset of late Cenozoic polar cooling. The evidence is that (1) extant kelps occur exclusively in cold-water habitats; (2) all but one of 27 kelp genera occur in the North Pacific, 19 of these exclusively; and (3) limpets and herbivorous marine mammals obligately associated with kelps or other stipitate brown algae appeared late in the Cenozoic, even though more generalized forms of both groups are much older. We propose, further, that sea otters and perhaps other groups of benthic-feeding predatory mammals, whose late Cenozoic distributions all were limited to the North Pacific, created an environment for the evolution of kelps in which the intensity of herbivory was unusually low. We hypothesize that this interaction created predictable differences among habitats in the intensity of herbivory on several spatial scales, with resulting trade-offs between anti-herbivore defenses and plant competitive abilities in their respective floras. Sea otters incur time and energy costs for diving, resulting in depth-related reductions to foraging efficiency and thus increased sizes and densities of herbivorous sea urchins. Thus, the deep-water flora is well defended, but competitively subordinate, compared with the shallow-water flora. Similarly, we argue that during the same period of earth history, predation had less of a limiting influence on herbivorous invertebrates in the temperate southwestern Pacific. We hypothesize that (1) consequent biogeographical differences in the intensity of herbivory may have selected the phenolic-rich brown algal flora in temperate Australia/New Zealand; and (2) tightly coevolved plant/herbivore interactions may explain why Australian and New Zealand herbivores are undeterred by phenolics and why other classes of secondary compounds in the Australian/New Zealand flora significantly deter herbivores.