Water and Existence: The Cosmic and Planetary History of H₂O from the Big Bang to the Present

Water is both a molecular product of cosmic chemical evolution and a dynamic component of planetary systems. This synthesis reconstructs the history of H₂O across five linked levels: primordial hydrogen, stellar production of oxygen, interstellar and circumstellar water chemistry, incorporation of volatiles into the forming Solar System, and the subsequent evolution of Earth's surface and deep water reservoirs. The central methodological point is that "the origin of Earth's water" is not equivalent to a single delivery event. Hydrogen and oxygen have different nucleosynthetic histories; water molecules can form before, during, and after star and planet formation; and Earth's present water inventory reflects accretion, devolatilization, atmospheric loss, interior storage, surface condensation, and long-term recycling. Isotopic evidence, especially D/H, provides powerful constraints but does not support a single deterministic source percentage. Carbonaceous asteroids/chondrites are strongly implicated in the volatile inventory of the inner Solar System, while some comets possess Earth-like D/H values and therefore cannot be excluded categorically. Deep-Earth water storage is also treated cautiously: mineral capacity and local hydrous evidence do not establish a globally ocean-equivalent inventory. The paper concludes with an evidence-based framework linking cosmic chemistry, planetary accretion, the hydrological cycle, deep-Earth exchange, biospheric evolution, and anthropogenic water use. The result is a scientifically conservative narrative designed to distinguish observation, laboratory constraint, inference, model dependence, and conceptual synthesis.