Adenosine-to-inosine (A-to-I) RNA editing, capable of protein recoding, has evolved independently in animals and fungi. This study proposes adaptive hypotheses regarding its origins and phenotypic significance, suggesting that A-to-I editing enhances adaptability by alleviating genetic trade-offs. In metazoans, its emergence may have been driven by a development-defense trade-off associated with transposable element activation during the evolution of multicellularity. Late Devonian cooling and End-Permian warming are hypothesized to have driven the emergence of extensive A-to-I recoding in coleoid nervous systems and Sordariomycete sexual fruiting bodies, respectively. These adaptations may have influenced key evolutionary innovations, including the evolution of metazoan nervous systems, coleoid intelligence, and shell loss, and fungal sexual reproductive structures. Additionally, extensive A-to-I recoding is proposed to facilitate accelerated development and specific life-history strategies in both animals and fungi. This paper provides new perspectives on the evolutionary forces shaping A-to-I RNA editing and its role in phenotypic diversity across taxa.