| Summary: | The purpose of this study is to reconstruct an environmental context for the emergence and expansion of early animal life during the Middle Proterozoic (∼1.8–0.8 billion years ago). Specifically, we examined the spatially and temporally evolving Earth surface ocean oxygen (O2) concentration impacted by atmospheric partial pressure of oxygen (pO2) and phosphorus levels. We exploited a quantitative biogeochemical model - the Grid ENabled Integrated Earth system (cGENIE) model, which consists of a reduced physics 3-D ocean circulation model coupled to a 2-D energy-moisture balance model of the atmosphere and a dynamic-thermodynamic sea-ice model. Our results suggest a challenging evolutionary landscape for basal metazoan life characterized by spatiotemporal variation in surface ocean oxygen level and prevalent benthic anoxia even with surface ocean-atmosphere oxygen level sufficient to support basal biological activities. By studying the environmental variability presented during the evolutionary history of complex life on Earth, we are thus equipped with a valuable tool to examine the potential biotic complexity throughout the universe. Undergraduate
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