The MUT-7 family of 3’-5’ exoribonucleases is evolutionarily conserved across the animal kingdom and plays essential roles in small RNA production in the germline. Most MUT-7 homologs carry a C-terminal domain of unknown function named MUT7 C appended to the exoribonuclease domain. Our analysis shows that the MUT7-C is evolutionary ancient, as a minimal version of the domain exists as an individual protein in prokaryotes. Throughout evolution, MUT7-C has acquired different insertions in animals, expanding its functions. C. elegans MUT-7 contains a specific insertion within MUT7-C, which allows binding to MUT 8 and, consequently, MUT-7 recruitment to germ granules. In addition, in C. elegans and human MUT-7, the MUT7 C domain contributes to RNA binding and is thereby crucial for nuclease activity. This RNA-binding function most likely represents the ancestral function of the MUT7 C domain. Overall, this study sheds light on MUT7-C and assigns two functions to this previously uncharacterised domain.