Every molecule of vitamin B12 in your bloodstream was, at some point in its history, built by a microbe. Not by a cow. Not by a chicken. Not by a spinach leaf or a sunflower or any plant that has ever grown on Earth. Only bacteria and archaea — the two oldest branches of life — carry the full genetic machinery to assemble cobalamin, the cobalt-cored ring that keeps your nerves insulated and your r...