An exception might be the Namib Desert, which has been proposed to be as old as 65 Ma (Ward and Corbett, 1990). This comparatively greater age may explain why the Namib desert is the only hot desert that is highly diverse (Barthlott et al., 2005). Because of their recent origin, many desert plant species are in “younger” branches of the tree of life, resulting in lower phylogenetic diversity when compared to relatively more humid environments. Thus, if we consider the relative contribution of desert taxa to PD compared to other vegetation types (tropical, temperate, and/or Mediterranean), hot deserts are likely to appear as regions of low PD. Keywords: arid environments, Australia, California, Chile, complementarity, conservation priorities, neoendemism, option value, phylogenetic diversity, phylogenetic endemism.