The origin and evolution of land plants (i.e., embryophytes) represents one of the seminal events in the history of life on earth. When the land plant lineage took hold upon the continents, there were profound changes in the global environment including dramatic modification of the erosion regime, correlated changes in marine nutrient inputs, and striking fluctuations in atmospheric chemistry. These changes took place not only because of the ability of land plants to survive on land, but also because their structure and physiology permitted them to make efficient use of resources, anchor themselves effectively and in so doing stabilize the substrate and permit the formation of modern soils, and maintain metabolic activity for long periods of time even in the absence of rainfall and surface moisture. Molecular phylogenetic analyses show the order Charales is the sister group to all land plants, with the Coleochaetales sister to the land plant/Charales lineage. Thus in a very real sense, the embryophytes are “drier algae.” Many groups of green algae live in the terrestrial environment, but only one of these – the land plants – has radiated into a wide range of habitats. It is not known why the land plant lineage has been so successful. Traits that are likely to have played a role in their success include cell wall biochemistry, desiccation resistance and tolerance, structural complexity, and various reproductive strategies. In all probability the success of the land plant lineage was not the result of a single “key innovation,” but an emergent property resulting from complex interactions among these and other features of the lineage. Comparative study of the properties of diverse aquatic and terrestrial algae can be used to identify properties that were important in the colonization of the land and how they interact.

Key words: Charophyta, Chlorophya, Embryophyte, land plant, molecular phylogenetics, paleobotany