The production and utilization of oxygen supports most life on the modern Earth. Indeed, most modern animals need quite elevated amounts and would not survive in the Earth’s atmosphere 700 million years ago. In this talk I will trace the key developments in the history of atmospheric oxygen on Earth. These developments will be placed in the context of the evolving biosphere with a focus on interactions between oxygen and Eukaryote evolution. I will place special emphasis on the evolution of early animals and use physiological experiments to constrain how their evolution was impacted by the history of oxygen. I will also discuss how sponges cycle oxygen internally, providing possible clues as to how animals have evolved to both cope with and exploit the elevated levels of atmospheric oxygen we enjoy today.