Land plants evolved from freshwater charophyte algal ancestors during a single transition to the terrestrial environment. The six major lineages of land plants are divided into two groups: the bryophytes (liverworts, mosses, and hornworts) and the tracheophytes (lycophytes, ferns, and seed plants); while the tracheophytes are thought to be monophyletic, the bryophytes have typically been considered as the direct ancestors of tracheophytes and therefore an artificial, nonmonophyletic, group. Here, the molecular phylogenetic evidence for relationships is reviewed and evaluated, especially in-light of large genome-level studies that have been completed in the last few years. Consideration is given to evaluate competing hypotheses with respect to the underlying evolutionary assumptions of the models used to analyze the molecular data, and the degrees of support for particular hypotheses. It is concluded that currently the two better-supported hypotheses are, firstly, that the bryophytes are a monophyletic group, and secondly, and less likely, that a lineage consisting of liverworts and mosses branched first among land plants with the hornworts the most-closely related lineage to tracheophytes. Although hitherto rarely considered, the possible monophyly of bryophytes has important implications for the morphological reconstruction of the last common ancestor of all land plants. Indeed, it might suggest that the ancestor of land plants was vascularized and had alternating generations that were more isomorphic than is found in extant taxa. The evolution of the bryophytes might then have proceeded through elaboration of the gametophyte and reduction of the sporophyte, while the opposite being true of the tracheophyte lineage.
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