Figure 2: Timeline of Metatherian and Eutherian mammalian evolution showing the emergence of TRIM22. The divergence of Metatherian (marsupial) and Eutherian (placental) mammals occurred approximately 180 million years ago in the Jurassic period of the Mesozoic era. The TRIM22 gene emerged sometime after this division, as it is absent in Metatherian mammals but present in all major Eutherian groups. In addition, since TRIM22 is present in all Eutherian mammals, it must have emerged before further Eutherian division occurred (approximately 90 million years ago). Taken together, this dates the birth of TRIM22 to approximately 90–180 million years ago. The predicted window of time for TRIM22 emergence in Eutherian mammals is demarcated with two red arrows. MYA: millions of years.