Corals build the structural foundation of coral reefs, one of the most diverse and productive ecosystems on our planet. While the process of coral calcification that allows corals to build these immense structures has been extensively investigated, we still know little about the evolutionary processes that allowed the soft-bodied ancestor of corals to become the ecosystem builders they are today. Using a combination of phylogenomics, proteomics and immunohistochemistry, we show that scleractinian corals likely acquired the ability to calcify sometime between ∼308 - ∼265 Mya through a combination of lineage specific gene duplications and the co-option of existing genes to the calcification process. Our results suggest that coral calcification did not require extensive evolutionary changes, but rather few coral-specific gene duplications and a series of small, gradual optimizations of ancestral proteins and their co-option to the calcification process.