The ocean has been a feature of Earth’s surface for at least four of the past 4.5 billion years and has provided the primary environment for the evolution of microbes that drive Earth’s biogeochemical cycles (Falkowski et al., 2008). Over this incomprehensively long time period, the ocean and the organisms in it have witnessed extreme changes, ranging from complete coverage with ice to extensive periods when there was no ice at all. There have been periods of extraordinary extinction of animal life due to meteorite impacts and volcanic outgassing, when the ocean became acidic and anoxic for extensive periods of time, and long intervals of relative stability that fostered the evolution of animals, from which we ultimately descend. Yet most of us never think about how the organisms that drive the biogeochemical cycles in the ocean evolved and have survived these extreme environmental changes to provide the backbone of life on Earth. Indeed, microbes in general, and marine microbes in particular, are the real stewards of life on Earth. We have a lot to learn about how they work and function to make this planet habitable. In this article, I examine how life evolved in the ocean, how it impacted the evolution of mammals, including humans, and how we are impacting the ocean.