Life on Earth created, and is dependent on, nonequilibrium cycles of electron transfers involving primarily five elements: hydrogen, carbon, nitrogen, oxygen, and sulfur (1). Although biophysical and biochemical reactions catalyze specific electron transfers at a local, molecular level, the metabolic consequences are global. Through opportunity and selection, metabolic pathways evolved to form an interdependent, planetary "electron market" where reductants and oxidants are traded across the globe. The exchanges are made on a planetary scale because gases, produced by all organisms, can be transported around Earth's surface by the ocean and atmosphere. Exactly how these five elements came to form an electron market place remains largely unresolved, however. On page 1764 of this issue, Raymond and Segre (2) use an ingenious bioinformatics approach to reveal the evolution of metabolic pathways. Their analysis elegantly reveals not only the profound role that molecular oxygen (O_2) has played in shaping the electron market place, but also the evolutionary constraints on, and trajectories of, the ensemble of electron traders.
展开▼
