Deserts are not usually considered biodiversity hotspots, but desert microbiotic crust communities exhibit a rich diversity of both eukaryotic and prokaryotic life forms. Like many communities dominated by microscopic organisms, they defy characterization by traditional species-counting approaches to assessing biodiversity. Here we use exclusive molecular phylodiversity (E) to quantify the amount of evolutionary divergence unique to desert-dwelling green algae (Chlorophyta) in microbiotic crust communities. Given a phylogenetic tree with branch lengths expressed in units of expected substitutions per site, E is the total length of all tree segments representing exclusively desert lineages. Using MCMC to integrate over tree topologies and branch lengths provides 95% Bayesian credible intervals for phylodiversity measures. We found substantial exclusive molecular phylodiversity based on 18S rDNA data, showing that desert lineages are distantly related to their nearest aquatic relatives. Our results challenge conventional wisdom, which holds that there was a single origin of terrestrial green plants and that green algae are merely incidental visitors rather than indigenous components of desert communities. We identify examples of lineage diversification within deserts and at least 12 separate transitions from aquatic to terrestrial life apart from the most celebrated transition leading to the embryophyte land plants. [Bayesian phylogenetics; biodiversity; exclusive molecular phylodiversity; microbiotic crusts.].