Preadaptivity of Noncontractile Thermogenesis in the Evolution of Warm-Bloodedness in Vertebrates

Abstract

Most of the heat that is released in the vertebrate body is produced in the muscles during contractile (during movement or trembling) and noncontractile (without muscle activity) thermogenesis. Contractile thermogenesis is characteristic of all vertebrates, but it is not able to maintain a constantly high body temperature in animals. The main idea discussed in this article and based on a large number of publications of recent years is as follows: the main biochemical basis of warm-bloodedness in vertebrates is part of the cycle of contraction–relaxation of striated skeletal muscles, in which the act of muscle contraction somehow falls out, and the energy that should have been used for it is dissipated in the form of heat. This noncontractile thermogenesis, which is able to support the regional and general endothermy in vertebrates, can be considered the real biochemical basis of warm-bloodedness. Thus, the presence of skeletal muscles in all vertebrates and the common biochemical foundations of the contraction–relaxation cycle represent a single preadaptive property of the manifestation of noncontractile thermogenesis in all vertebrates, starting with fish, which is a basis for the evolution of warm-bloodedness. Therefore, the modern data that the first terrestrial vertebrates were most likely animals with high levels of both metabolism and body temperature are quite understandable and not surprising.