Abstract <p>The article describes a new characteristic that is important in the process of evolution of vertebrates—the quality of activity. This is a set of properties that primarily describes the motor activity of animals and manifests itself in the energy supply of activity and work of the nervous system (and at all levels—from oxidation in the mitochondria to the basic metabolism of the whole organism), in speed, in coordination and accuracy of movements, in optimizing purposeful, expedient behavior owing to the activity of the central nervous system, and in endurance and other properties that give animals selective advantages in competition with other animal groups, which create prerequisites for their ecological and evolutionary success. This characteristic is important primarily for vertebrates. Improving the quality of activity is the most important direction in the evolution of vertebrates. It indirectly includes the evolutionary characteristics of a multitude of internally interconnected functional systems of the body (musculoskeletal system, energy system from molecular to ecological levels, circulatory and respiratory systems, central nervous system, behavior, etc.) and is thus an integral characteristic responsible for the interrelation of the vital activity of the body with the external environment, for its adaptations, and for its interaction with cenoses of different levels. In our opinion, the quality of activity is a key characteristic, which ultimately is the “axis” around which the evolution of the complex of vertebrates is formed, and which is the basis, the direction of natural selection in this group of animals, ensuring the improvement of all other functionally interconnected internal systems of the body. This crucial characteristic requires further study and development.</p>