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Temperature is one of the most fundamental environmental parameters influencing biochemical reactions and the structural stability of biological macromolecules. Enzymes, which act as catalytic proteins in metabolic systems, exhibit a strong dependence on temperature due to the combined effects of molecular kinetics and protein thermodynamics. At low temperatures, reduced molecular motion limits the frequency and efficiency of enzyme–substrate collisions, resulting in slow reaction rates. As temperature increases, catalytic activity rises because of enhanced kinetic energy and increased probability of productive molecular interactions. However, this enhancement occurs only within a restricted thermal interval. At temperatures exceeding a critical threshold, the weak non-covalent interactions that maintain protein structure—including hydrogen bonds, ionic interactions, and hydrophobic forces—become destabilized, leading to protein unfolding and loss of enzymatic function through thermal denaturation. This dual influence of temperature produces a functional “thermal window” in which enzymatic catalysis can operate efficiently only within a specific temperature range bounded by kinetic limitations at low temperatures and structural instability at high temperatures. The present conceptual analysis proposes the Thermal Window Hypothesis, which suggests that early biochemical systems could function only within a limited temperature range where enzyme catalytic activity and protein structural stability were simultaneously maintained. Because primitive enzymes were likely less structurally stabilized than modern proteins, early metabolic systems would have been particularly sensitive to environmental temperature fluctuations. Consequently, the emergence and persistence of early life were likely favored in environments characterized by moderate and relatively stable thermal conditions. This framework provides a biochemical perspective on environmental constraints relevant to the origin of life, suggesting that thermally buffered habitats—such as shallow aquatic systems or geothermally moderated environments—may have provided favorable conditions for the development of early metabolic processes. Understanding the thermal limits of enzyme function therefore offers valuable insight into the environmental parameters that may have shaped the earliest stages of biological evolution.