All life on earth requires a source of energy and organic carbon. There has been a continuous debate on whether autotrophic or heterotrophic metabolism came first. A very similar discussion exists concerning the advent of oxygenic photosynthesis and aerobic respiration. I put forward the synchronistic evolution hypothesis supposing that all metabolic processes develop in a bidirectional manner from the very first. Bidirectionality is claimed to be intrinsic to the evolution of all metabolic processes as (i) all biochemical reactions and enzymes are per se bidirectional, (ii) substrates need to be regenerated, and (iii) flux regulation requires flexibility of direction. Autotrophy and heterotrophy are thus inherent to each other. A scenario for the synchronistic development of oxygenic photosynthesis and aerobic respiration is described.