Implications of Abiotic Oxygen Buildup for Earth-like Complex Life

One of the chief paradoxes of molecular oxygen (O₂) is that it is an essential requirement for multicellular eukaryotes on Earth while simultaneously posing a threat to their survival via the formation of reactive oxygen species. In this paper, the constraints imposed by O₂ on Earth-like complex life are invoked to explore whether worlds with abiotic O₂ inventories can harbor such organisms. By taking the major O₂ sources and sinks of Earth-like planets into account using a simple model, it is suggested that worlds that receive time-averaged X-ray and extreme ultraviolet fluxes that are ≳10 times higher than Earth might not be capable of hosting complex lifeforms because the photolysis of molecules such as water may lead to significant O₂ buildup. Methods for testing this hypothesis by searching for anticorrelations between biosignatures and indicators of abiotic O₂ atmospheres are described. In the event, however, that life successfully adapts to high-oxygen environments, these worlds could permit the evolution of large and complex organisms.