ABSTRACT
Hydrogen has played an important role in Earth’s geochemistry and biology since the earliest stages of the planet’s history. H2 is postulated to have been a significant component of the early, prebiotic atmosphere, where it had direct or indirect effects on atmospheric redox chemistry and radiation budget, and contributed to planetary oxidation by escape to space. With the origins and evolution of life came a broad variety of metabolisms involving H2, and an expanded role for H2 in planetary biogeochemistry. H2 levels in the modern atmosphere are perhaps three orders of magnitude or more lower than during Earth’s earliest history, but the significance of H2 cycling for planetary chemistry persists through its central role in the microbial world.
