Functional Directions of the Development of the Heart in Vertebrates in Connection with the Evolution of Their Thermoenergetic Status

The main evolutionary direction in vertebrates is to improve energy supply and activity quality. During the evolution of vertebrates, a functional orientation towards an increasing separation of arterial and venous blood flows is observed in their hearts. Fish have a two-chambered heart and amphibians have a three-chambered heart, which fully provides all their energy needs. However, basal amphibian-like tetrapods experienced a sharp, abrupt increase in the levels of mitochondrial oxidation and basal metabolism about 340 million years ago. This significantly improved their energy supply and activity quality and allowed them to go onto land and adapt to the new environment. This led to the appearance of mesometabolic basal terrestrial tetrapods, i.e., the first reptiles (amniotes). Their thermometabolism then also increased, which endogenously increased their body temperature. Two directions of development of thermoenergetic statuses are observed in vertebrates: (1) the development of tachymetabolic psilothermic endothermy, i.e., warm–bloodedness (the peaks of development are birds in the sauropsid clade and mammals in the synapsid clade); their heart is completely four-chambered; and the heart of tachymetabolic, warm–blooded crocodylomorph reptiles were inherited by modern bradymetabolic crocodiles; (2) the development of bradymetabolic psilothermic ectothermy, i.e., relative cold-bloodedness (the peak development is represented by all modern reptiles); the heart of mesometabolic reptiles became incompletely five-chambered, in which arterial and venous blood flows were separated simultaneously with their controlled mixing, which made it possible to regulate the level of metabolism; these hearts were inherited by their bradymetabolic descendants, modern lepidosaurs and turtles.