Galactic Inheritance in the Breathing Universe

Life on Earth began with a chemical dowry it did not make: sugars, nucleobases, amino acids, phosphate, and the precursors of membranes, much of it assembled in interstellar space and delivered ready-made. This is a synthesis of what is known about that galactic inheritance -- which life-relevant molecules exist across the Milky Way, how their abundance and complexity are distributed, how they travel from molecular cloud to planet, when in galactic history they became available, and how the Earth-bound record of meteorites and returned samples confirms the account. The distribution is markedly non-uniform: the raw materials thin with galactocentric radius, complex prebiotic molecules concentrate dramatically toward the galactic center, the realized inventory varies by environment, and the census itself is biased by where sensitive telescopes have looked -- yet the grain-surface chemistry that builds these molecules is generic to any cold, dense cloud. The delivery chain -- cloud to protoplanetary disk (largely by inheritance, on the evidence of deuterium fractionation), to comets and carbonaceous asteroids, to planets by impact and by the steady rain of interplanetary dust -- is traced together with the galactic-chemical-evolution timing that made it possible. The placement of this inheritance on the cosmic arc, and its thermodynamic reading, are developed in the companion paper (RNA and DNA in the Breathing Universe, DOI 10.5281/zenodo.21878770), to which this paper is the prehistory; here the focus is the observational and cosmochemical record itself.