From Water to Land and Back

This narrative literature review examines the evolutionary relationship between aquatic ancestry, terrestrial adaptation, and the secondary return of certain vertebrate lineages to aquatic environments, with particular attention to birds, penguins, humans, and cephalopods. Drawing upon published research in evolutionary biology, paleontology, comparative physiology, and bioinorganic chemistry, the review investigates the survival of avian dinosaurs following the Cretaceous–Paleogene extinction, the independent evolution of aquatic adaptations, and the biochemical differences between iron-based hemoglobin and copper-based hemocyanin. Particular attention is given to oxygen binding, oxidation states, environmental salinity, temperature, pH, and the physiological mechanisms governing oxygen transport. The review further examines documented copper–iron interactions in mammalian metabolism alongside the electrochemical principle of sacrificial corrosion protection, in which metallic iron may preferentially oxidize and thereby protect metallic copper. These established phenomena are distinguished from the unverified hypothesis that iron availability could influence the structural stability or oxygen-binding function of cephalopod hemocyanin. A preliminary experimental approach is proposed to investigate this relationship without presupposing a protective mechanism. The review concludes that evolutionary continuity and shared biochemical requirements can produce markedly different physiological adaptations. It identifies further research questions concerning metal-dependent oxygen transport and the relationship between biological oxidation processes and inorganic electrochemical behavior. Research classification: Narrative literature review and preliminary hypothesis development. Methodological disclosure: This document was developed through AI-assisted synthesis of published scientific literature. It does not report original experimental findings and has not undergone independent peer review.