The evolution of oxygen, temperature, and marine aerobic habitat for animals over the last billion years of Earth history.

Atmospheric and ocean oxygen levels and temperature have changed considerably over Earth history, and many of these changes coincided with similarly dramatic changes in marine life. The synergistic interaction between oxygen availability and temperature sets aerobic capacities for marine animals, determining whether a given environment is habitable. Here, we explore the relationship between oxygen, temperature and aerobic habitability across the last billion years of Earth history using a metric known as the 'metabolic index' (Φ). Idealized scenarios for oxygen-temperature trajectories indicate several distinct avenues for a concurrent transition from 'non-permissive' to 'permissive' aerobic environments at the onset of the Cambrian Period. Thereafter, the continued presence of fossils in shallow, tropical habitats sets clear constraints on oxygen-temperature combinations, suggesting that the most extreme published estimates for each are improbable. We then draw on published oxygen and temperature estimates to illustrate how these variables have controlled aerobic habitability for marine animals over the Phanerozoic. The dominance of hypoxia-tolerant faunas during the Paleozoic is explained in part by low oxygen oceans, while increasing oxygen levels in the Devonian likely led to lower extinction rates. Sharp spikes in temperature, such as during the end-Permian, led to the contraction of aerobic habitat and mass extinction, but probably also to a more temperature-tolerant modern fauna. We emphasize the need for more robust environmental records of early animal evolution and a better understanding of trade-offs in animal physiology. Finally, exciting future applications of the metabolic index to astrobiology have arisen as knowledge of extraterrestrial environmental conditions increases.