Over the past four billion years, life on earth has evolved from hydrogen breathing, to sulfur breathing, and eventually to oxygen breathing. Underlying these dramatic changes is the evolution of the energy-conserving respiratory complexes, from membrane-bound hydrogenase (MBH), to membrane-bound sulfane reductase (MBS), and finally to the modern-day Complex I. Dr. Li will describe their structural analyses of MBH and MBS isolated from the membranes of extreme thermophile Pyrococcus furiosus, and compare these structures with the modern-day Complex I, highlighting a key structural feature nature has evolved to enable energy transduction from the redox potential to chemical gradient.