The emergence of intracellular calcification among marine unicellular eukaryotes in the Late Triassic (237-201 Ma) had profound consequences for the carbonate buffering capacity of the ocean. Research on the appearance of calcification typically focuses on the reasons that made this process successful on a global scale. The underlying environmental and physiological conditions that led to its appearance, therefore, are still obscure. Using gene tree analysis, we show that the physiological machinery for calcification was already present in non-calcifying ancestor cells. Additionally, by modelling the energy demands for calcium transport in calcifying and non-calcifying cells, we demonstrate that intracellular calcification does not require additional energy investments. Since all eukaryotic cells export calcium across the plasma membrane, our findings indicate that the onset of intracellular calcification in Earth history only required the activation of calcium transport proteins during their passage to the plasma membrane. Our work sheds new light on the physiological and biogeochemical conditions that led to one of the most important evolutionary innovations of the Mesozoic era.