New research from Stockholm University with participation of the Museum für Naturkunde Berlin shows that the typical mammalian heater organ, brown fat, evolved exclusively in modern placental mammals. In collaboration with the Helmholtz Munich and the University of East Anglia in the U.K., the Stockholm University research team demonstrated that marsupials, our distant relatives, possess a not fully evolved form of brown fat. They discovered that the pivotal heat-producing protein called UCP1 became active after the divergence of placental and marsupial mammals. This finding is crucial for understanding the role of brown fat in mammalian evolution, endothermy, and metabolism.