The main energy-transducing metabolic systems originated and diversified very early in the evolution of life. This makes it difficult to unravel the precise steps in the evolution of the proteins involved in these processes. Recent molecular data suggest that homologous proteins of aerobic respiratory chains can be found in Bacteria and Archaea, which points to a common ancestor that possessed these proteins. Other molecular data predict that this ancestor was unlikely to perform oxygenic photosynthesis. This evidence, that aerobic respiration has a single origin and may have evolved before oxygen was released to the atmosphere by photosynthetic organisms, is contrary to the textbook viewpoint.