Gene duplication-divergence and enzyme promiscuity drive metabolic diversification in plants, but how they contribute to functional innovation in enzyme families is not clearly understood. In this study, we addressed this question using the large BAHD acyltransferase family as a model. This fast-evolving family, which uses diverse substrates, expanded drastically during land plant evolution. In vitro characterization of 11 BAHDs against a substrate panel and phylogenetic analyses revealed that the ancestral enzymes prior to origin of land plants were likely capable of promiscuously utilizing most of the substrate classes used by current, largely specialized enzymes. Motif enrichment analysis in anthocyanin/flavonoid-acylating BAHDs helped identify two motifs that potentially contributed to specialization of the ancestral anthocyanin-acylation capability. Molecular dynamic simulations and enzyme kinetics further resolved the potential roles of these motifs in the path towards specialization. Our results illuminate how promiscuity in robust and evolvable enzymes contributes to functional diversity in enzyme families.