How did the first chemical reactions get started at the origin of life and what was their source of energy? Researchers have reconstructed the metabolism of the last universal common ancestor, LUCA. They found that almost all chemical steps used by primordial life to piece together the molecular building blocks of cells are energy releasing reactions. This identified the long-sought source of energy needed to drive these reactions forward, which has been hiding in plain sight. The energy required to synthesize the building blocks of life comes from within metabolism itself, as long as one essential starting compound is included. The secret ingredient that releases the energy from within at life's origin is the cleanest, greenest, newest and oldest of all energy carriers: Hydrogen gas, H2.