Proterozoic evolution of the phosphorus cycle: Was it high or was it low?

The Proterozoic eon witnessed the emergence of eukaryotes and the protracted oxygenation of the Earth's surface. In that regard, phosphorus, considered the limiting nutrient on geologic timescales, may have played a substantial role, as it links to the biogeochemical cycle of carbon and to the evolution of life. However, reconstructing its bioavailability through the Proterozoic is difficult, as it relies on limited evidence from the rock record, with no direct estimation of seawater phosphorus levels. Here, I will review and discuss current debates around this issue and try to identify the main limitations of our understanding of the Proterozoic phosphorus paradigms.