The evolution of life is intricately linked to the evolution of Earth's geochemical cycles. In particular, the connections between ancient life and the biogeochemical cycles of carbon and oxygen have been intensively scrutinized because these elements are fundamental components of organic matter, determine the pathways through which organisms can obtain energy, and are critical for defining habitability. Despite this attention, significant questions remain about how the evolution of life is intertwined with environmental conditions. To this end, I analyze uranium (U) and calcium (Ca) isotopes in carbonate rocks and reconstruct two major processes that control carbon and oxygen in the ocean-atmosphere system: the degree of ocean oxygenation and variations in carbonate chemistry. These isotopic records can also constrain the carbon isotope record, the predominant tool for understanding the past carbon cycle. Here, I focus on fluctuations in marine environmental conditions during two critical time intervals in the history of animals in order to understand the primary factors governing marine biogeochemical cycles: the Cryogenian non-glacial interval (660 to 640 Ma) and the recovery from the end-Permian mass extinction (252 to 235 Ma). Combined, these studies employ non-traditional isotopic approaches to investigate how global patterns of environmental change, evolution, and extinction are related. Indeed, the results described herein emphasize that critical events in the history of animals are accompanied by dramatic variations in biogeochemical cycles and environmental conditions.