A circulation-based hypothesis for vertebrate body plan evolution and endothermy

The emergence of novel vertebrate body plans and endothermy remain central questions in evolutionary science. Although gene regulatory networks provide powerful explanatory frameworks, the contribution of systemic physiology to vertebrate evolution warrants further investigation. Here, a hypothetical circulation-based model is proposed that integrates the redox and magnetic properties of blood with circulatory architecture. The circulation is modelled as a redox-balanced, one-way oxygen delivery scaffold that broadly reflects the vertebrate segmental body plan. It is hypothesized that an oxygenated systemic shunt within an ancestral, fish-like single-loop circulation generates a kinematically coupled ‘loop-in-loop’ dynamic. The evolvable scaffold adjusts to maintain a balanced state of redox eustress, influencing cardiorespiratory topology and the disposition of intersegmental blood flow. With greater air breathing dependence and strengthening of the loop-in-loop, transition from this archetypal tetrapodal circulatory format to the double-looped circuits of mammals and birds is achieved. This circulation-based ecophysiological framework is examined in relation to extant vertebrate anatomy and comparative evolutionary and developmental evidence. Planck’s equation is used in calculating the thermogenic potential associated with heme oxygenation within the lungs as compared to the gills or with cutaneous oxygenation. The model is restricted to the physical properties of circulating blood and does not explicitly incorporate mitochondrial metabolism, genomic regulation, or other biological processes. It is presented as a testable hypothesis rather than a causal explanation of vertebrate evolution. If supported by empirical investigation, this framework may complement existing evolutionary and developmental models by identifying potential physiological mechanisms relating to vertebrate body plan organization and thermal physiology. This knowledge may add to predicting how organisms may respond to climate change and may provide further insights into evolutionary medicine and developmental disorders.