Analysis of DNA polymerase iota (Pol iota) enzymic activity in different classes of eukaryotes has shown that error-prone activity of this enzyme can be found only in mammals, and that it is completely absent from organisms that are at lower stages of development. It was supposed that the emergence of the error-prone Pol iota activity in mammals is caused by structural alteration of the active center. Possible functions of error-prone Pol iota in higher eukaryotes are discussed.