Islands are or have been occupied by unusual species, such as dwarf proboscideans and giant rodents. The discussion of the classical, but controversial “island rule,” which states that mammalian body sizes converge on intermediate sizes on islands, has been stimulated by these unusual species. In this study, we use an unprecedented global data set of the distributions and body sizes of late-Quaternary mammal species and a novel analytical method to analyze body size evolution on islands. The analyses produced strong support for the island rule. Islands have suffered massive human-driven losses of species, and we found that the support for the island rule was substantially stronger when the many late-Quaternary extinct species were also considered (particularly, the tendency for dwarfing in large taxa). The decisive support for the island rule in this study confirms that evolution plays out markedly different on islands and that human impact may obscure even fundamental evolutionary patterns.