Eukaryotic cells generate cytosolic Ca2+ signals via Ca2+-conducting channels in cellular membranes. Plants and animals exhibit substantial differences in their complement of Ca2+ channels. In particular, the four-domain voltage-dependent Ca2+ channels, transient receptor potential channels and inositol (1,4,5)-trisphosphate receptors, which have important roles in animal physiology, are all absent from land plants. Recent evidence from biochemical and genomic studies has indicated that representatives of these classes of Ca2+ channels are present in members of the green plant lineage, the chlorophyte algae. This indicates that the Ca2+-signalling mechanisms absent from land plants were, in fact, present in ancestral eukaryotes and were lost by land plants after their divergence from the chlorophyte algae.