Why O2 is required by complex life on habitable planets and the concept of planetary "oxygenation time"

Life is constructed from a limited toolkit: the Periodic Table. The reduction of oxygen provides the largest free energy release per electron transfer, except for the reduction of fluorine and chlorine. However, the bonding of O-2 ensures that it is sufficiently stable to accumulate in a planetary atmosphere, whereas the more weakly bonded halogen gases are far too reactive ever to achieve significant abundance. Consequently, an atmosphere rich in O-2 provides the largest feasible energy source. This universal uniqueness suggests that abundant O-2 is necessary for the high-energy demands of complex life anywhere, i.e., for actively mobile organisms of similar to 10(-1)-10(0) m size scale with specialized, differentiated anatomy comparable to advanced metazoans. On Earth, aerobic metabolism provides about an order of magnitude more energy for a given intake of food than anaerobic metabolism. As a result, anaerobes do not grow beyond the complexity of uniseriate filaments of cells because of prohibitively low growth efficiencies in a food chain. The biomass cumulative number density, n, at a particular mass, m, scales as n (> m)alpha m(-1) for aquatic aerobes, and we show that for anaerobes the predicted scaling is n alpha m(-1.5), close to a growth-limited threshold. Even with aerobic metabolism, the partial pressure of atmospheric O-2 (PO) must exceed similar to 10(3) Pa to allow organisms that rely on O-2 diffusion to evolve to a size similar to 10(-3)m P-O2 in the range similar to 10(3)-10(4) Pa is needed to exceed the threshold of similar to 10(-2) m size for complex life with circulatory physiology. In terrestrial life, O-2 also facilitates hundreds of metabolic pathways, including those that make specialized structural molecules found only in animals. The time scale to reach P-O2 similar to 10(4) Pa, or "oxygenation time," was long on the Earth (similar to 3.9 billion years), within almost a factor of 2 of the Sun's main sequence lifetime. Consequently, we argue that the oxygenation time is likely to be a key rate-limiting step in the evolution of complex life on other habitable planets. The oxygenation time could preclude complex life on Earth-like planets orbiting short-lived stars that end their main sequence lives before planetary oxygenation takes place. Conversely, Earth-like planets orbiting long-lived stars are potentially favorable habitats for complex life.