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Evolutionary history

While the fossil record of crown group hornworts only begins in the upper Cretaceous, the lower Devonian Horneophyton may represent a stem group to the clade, as it possesses a sporangium with central columella not attached at the roof. However, the same form of columella is also characteristic of basal moss groups, such as the Sphagnopsida and Andreaeopsida, and has been interpreted as a character common to all early land plants with stomata. The divergence between hornworts and Setaphyta (mosses and liverworts) is estimated to have occurred 479–450 million years ago, and the last common ancestor of present-day hornworts lived in middle Permian about 275 million years ago. Chromosome-scale genome sequencing of three hornwort species corroborates that stomata evolved only once during land plant evolution. It also shows that the three groups of bryophytes share a common ancestor that branched off from the other landplants early in evolution, and that liverworts and mosses are more closely related to each other than to hornworts. Unlike other land plants, the hornwort genome has the low- CO inducible B gene (LCIB), which is also found in some species of algae. Because the diffusion rate of carbon dioxide is 10,000-fold higher in air than in water, aquatic algae require a mechanism to concentrate CO in chloroplasts so as to allow the photosynthetic RuBisCo protein to function efficiently. LCIB is one component of this CO -concentrating mechanism.