Brown adipose tissue serves as a thermogenic organ in placental mammals to defend body temperature in the cold by nonshivering thermogenesis. The thermogenic function of brown adipose tissue is enabled by several specialised features on the organ as well as on the cellular level, including dense sympathetic innervation and vascularisation, high lipolytic capacity and mitochondrial density and the unique expression of uncoupling protein 1 (UCP1). This mitochondrial carrier protein is inserted into the inner mitochondrial membrane and stimulates maximum mitochondrial respiration by dissipating proton-motive force as heat. Studies in knockout mice have clearly demonstrated that UCP1 is essential for nonshivering thermogenesis in brown adipose tissue. For a long time it had been presumed that brown adipose tissue and UCP1 emerged in placental mammals providing them with a unique advantage to survive in the cold. Our subsequent discoveries of UCP1 orthologues in ectotherm vertebrates and marsupials clearly refute this presumption. We can now initiate comparative studies on the structure-function relationships in UCP1 orthologues from different vertebrates to elucidate when during vertebrate evolution UCP1 gained the biochemical properties required for nonshivering thermogenesis.