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The largest proportion of microRNAs in humans (ca. 40–50%) originated in the phylogenetic grouping defined as primates. The dynamic evolution of this family of non-coding RNA is further demonstrated by the presence of microRNA unique to the human species. Investigations into the role of microRNA in cancer have until recently mainly focused on the more ancient members of this RNA family that are widely conserved in the animal kingdom. As I describe in this review the evolutionary young lineage and species-specific microRNA could be important contributors to cancers, especially in particular organs in primates compared to more distantly-related research models. Elucidating the biological significance of primate and human-specific microRNA in cancer could have important implications for cancer research and the use of non-primate animal models.The human miRNAome can be divided into miRNAs that are widely conserved, those that are primate-specific, and those that are human-specific. The latter two categories can also contribute to tumor development and complicate the study of human cancers in non-primate animal models.