Our results demonstrate that in addition to being used for controlling morphology during calcite growth, polysaccharide (PS) that has been designed for biomineralization is also extremely robust, influencing calcite reactions even after millions of years. We investigated calcite (CaCO₃) behavior in solutions with very small concentrations of PS that was produced ∼70 Ma ago by coccolithophorids. We used atomic force microscopy (AFM) and the constant composition method to monitor calcite growth in the presence of this ancient PS. The ancient PS is still very active and has a high affinity for calcite step edges. Adsorption, even at extremely low concentrations (0.5 μg/mL), results in decreased growth rate and dramatic morphology changes during growth and dissolution. The experimental results are complemented with surface complexation modeling for adsorption of components of polysaccharide from a modern coccolithophorid, Emiliania huxleyi. We generated surface complexation constants for the branch components: malonate: 14.25 ± 0.17, succinate: 11.91 ± 0.06, tricarballylate: 14.86 ± 0.04, and citrate: 15.25 ± 0.04. The implication is that complex PS could hold promise for smart material engineering and for preventing scaling. (Figure Presented).