We present a process of organic polyanion-mediated CaCO3 precipitation similar to biomineralization. Detailed insight into the formation pathway of nano-structured calcite crystals was gained through direct observation with an atomic force microscope (AFM). Essentially depending on a minimum saturation state, a primarily precipitated gelatinous film of polyaspartate calcifies as a transient, apparently amorphous phase, which gradually crystallizes as a calcitic single-crystalline nano-cluster of semicoherent domains. The similarity of this precipitation mechanism with that of biominerals and a striking consistency of materials properties of the synthetic precipitate with sea urchin calcite, investigated as a prime example of biomineralization, qualify our experiment as a functional model of biomineralization. The substantial role of water-soluble organic polyanions in biomineralization is elucidated.