During most of the eons when life has existed on earth, living organisms have been controlled in their activity not by their own needs but rather by the environment, in the simple way that life is based on chemical reactions; thus, with decreasing temperatures, activity decreases. The evolutionary advantages of a system that allows organisms to maintain a high and constant body temperature and thus to control their activity in accordance with their own demands is obvious, and different advanced species have attempted to approach this state. A major breakthrough occurred when development of the true mammals started. In an evolutionarily extremely rapid process, an existing mitochondrial transporter – with an unknown function – was completely changed so that it gained the ability, in a controlled manner, to allow for (the equivalent) of proton transport over the mitochondrial membrane. This allows for the release of the energy accrued by mitochondrial oxidation in the form of heat, instead of transferring it into ATP. Apparently, this development opposes the general concept of life as a system to conserve energy rather than utilize it, but this transformation of a proto-UCP1 to the mammalian UCP1 (uncoupling protein) provided the developing mammals with a totally new mechanism for gaining extra heat: that of nonshivering thermogenesis. We can only imagine that the intention to maintain a high and stable body temperature developed first and that initially the necessary extra heat could only be obtained through shivering, but those (pre)mammals that developed nonshivering thermogenesis obtained the ability to produce the extra heat in a comfortable way, that allowed them to do other things while heat was being produced imperceptibly in their bodies. In our opinion, no other mechanism for nonshivering thermogenesis exists in mammals except for that derived from the activity of UCP1 residing in the dedicated brown adipose tissue – and, to some extent, in certain nominally white adipose tissue depots (brite or beige adipose tissues). Possession of nonshivering thermogenesis allowed mammals to inhabit niches which were earlier closed for active life: the cold nights, the cold areas of the world, and thus gave them evolutionary advantages. Additionally, brown adipose tissue may have bestowed upon mammals the ability to perform so-called facultative diet-induced thermogenesis, a process in which some of the energy of the food eaten is combusted in an apparently meaningless way, but the process may allow mammals to extract important nutrients from rather low-quality food. As members of the mammalian group, we as humans share the ability to produce heat through brown adipose tissue activity. Until recently, this ability was thought to be restricted to newborn babies, but the present opinion is that brown adipose tissue is found and is active (when necessary) in a majority of adult humans as well. Thus, it may be said that the development of brown adipose tissue was what gave the developing mammals their evolutionary advantage, and that we still today as humans, even adults, are beneficiaries of this mammalian prerogative of possessing brown adipose tissue.