The carbonic anhydrases (CA) catalyze with high efficiency the reversible hydration of carbon dioxide, a reaction underlying many diverse physiological processes in animals, plants, archaebacteria, and eubacteria. We examined the evolutionary history and functional convergence of the CAs encoded by members of three independent CA gene families (alpha-CA, beta-CA and gamma-CA). Surprisingly, the six mammalian alpha-CA isozymes of defined function and tissue expression are evolving more rapidly than four mammalian alpha-CA-related proteins of unknown function. We have identified and included several previously unrecognized CA homologues present in the sequence databases, many of which are the fruits of genome project sequencing and expressed cDNA studies. We examined alpha-CA active site evolution and the putative beta-CA and gamma-CA active sites. We found support for the "introns late" hypothesis by analysis of alpha-CA intron locations. The view that alpha-CAs would be restricted to the animal kingdom and plant green algae (Chlamydomonas), the beta-CAs to plants and eubacteria, and the gamma-CAs to archaebacteria and eubacteria is breaking down. The plant Arabidopsis has homologues of all three families.