OXYGEN AND THE RISE COMPLEX LIFE

The long evolutionary history of life on Earth has unfolded as a cause-and-effect relationship with the changing levels of oxygen (O₂) in the oceans and atmosphere. Oxygen deficiency characterized our planet’s first two billion years, yet evidence for biological O₂ production and local enrichment in the surface ocean suitable for early aerobic metabolisms appears long before the first accumulations of O₂ in the atmosphere. The relationship between complex life and later oxygenation is less clear, although some data suggest that atmospheric O₂ remained low for another billion and a half years. This condition may have set a challenging course for complex life during its initial development and ecological expansion. The data suggest that O₂ stepped up again around 800 million years ago (Ma), coincident with a time of rapid eukaryotic innovation that included eukaryophagy (protists preying on protists). Multiple geochemical and paleontological records point to a major biogeochemical transition at that time, but whether and how rising biospheric O₂ triggered innovation in eukaryotic ecology, including the emergence of animals, is a matter of spirited debate. This talk will focus on the transition from the boring billion to a likely more oxygenated world around 800 Ma. We will emphasize diverse geochemical and paleontological records of concurrent biological and environmental change. This comparison reveals temporal and perhaps mechanistic relationships between dynamic biospheric oxygenation and coevolving life, including the early rise of animals.