Around 240 million years ago, during the late Triassic period, a crucial evolutionary development unfolded within the lineage that would give rise to mammals. It is believed that the XX/XY sex-determination system emerged in a common ancestor of mammals, possibly within the genus Therapsida, a group of synapsids that exhibited both reptilian and mammalian traits. This innovation in genetic sex determination provided a reliable mechanism for differentiating male and female individuals based on the presence of the Y chromosome, which carries the SRY gene responsible for initiating male development. This advancement played a pivotal role in the diversification and success of early mammalian ancestors.

While the XX/XY system is prevalent in mammals, it is not exclusive to them, and it evolved independently in different animal groups. Some reptiles, such as certain species of lizards and snakes, as well as some fish, including the medaka (Japanese rice fish), also employ this genetic mechanism for sex determination. However, in mammals, this system is universal. From tiny rodents to massive elephants, all mammals utilize the XX/XY system, ensuring males carry one X and one Y chromosome (XY), while females carry two X chromosomes (XX). This consistency has allowed mammals to thrive in a variety of environments, contributing to their evolutionary success. Even in monotremes like the platypus, which exhibit a more complex array of sex chromosomes, the fundamental principles of the XX/XY system are still at play, highlighting the enduring legacy of this ancient genetic innovation.