I reconstruct the Earth’s temperature record using a unique combination of field observations, micro-analytical, and geochemical methods. My data suggest that before the appearance of macroscopic animals and the evolution of modern phyla, climate was volatile and equatorial sea surface temperatures were elevated compared to the present. This observation goes against the consensus view that Earth’s mean temperature for the past three billion years was similar to today’s mean of 15°C. By exploring the ramifications of higher surface ocean temperatures, punctuated by extreme warming events and global glaciations, my work can unify a range of geologic and geochemical observations about the Precambrian Era, including the timing of the first appearance of macroscopic, multicellular animals; records of dissolved oxygen across different environments; changes in the volume, texture, and distribution of carbonate sedimentary rocks; and the occurrence of global glaciations.