Life's utilization of B vitamins on early Earth

Coenzymes are essential across all domains of life. B vitamins (B 1 ‐thiamin, B 2 ‐riboflavin, B 3 ‐niacin, B 5 ‐pantothenate, B 6 ‐pyridoxine, B 7 ‐biotin, and B 12 ‐cobalamin) represent the largest class of coenzymes, which participate in a diverse set of reactions including C 1 ‐rearrangements, DNA repair, electron transfer, and fatty acid synthesis. B vitamin structures range from simple to complex heterocycles, yet, despite this complexity, multiple lines of evidence exist for their ancient origins including abiotic synthesis under putative early Earth conditions and/or meteorite transport. Thus, some of these critical coenzymes likely preceded life on Earth. Some modern organisms can synthesize their own B vitamins de novo while others must either scavenge them from the environment or establish a symbiotic relationship with a B vitamin producer. B vitamin requirements are widespread in some of the most ancient metabolisms including all six carbon fixation pathways, sulfate reduction, sulfur disproportionation, methanogenesis, acetogenesis, and photosynthesis. Understanding modern metabolic B vitamin requirements is critical for understanding the evolutionary conditions of ancient metabolisms as well as the biogeochemical cycling of critical elements such as S, C, and O.