THE ORIGIN OF LAND PLANTS: A NEW LOOK AT AN OLD PROBLEM

Summary Although current ideas on evolution of the earth's atmosphere vary widely, it seems certain that when plants first invaded the land habitat they were exposed to solar ultraviolet radiation more intense than that found today. We suggest that an early obvious protective adaptation would be the accumulation of substituted cinnamic acids from the deamination of aromatic amino acids. If so, by making possible a diverse range of phenylpropanoid biochemistry, including the synthesis of lignin, this initial adaptation would have exerted a profound effect on the course of higher plant evolution. In/troduiction The sudden appearance of land plants is one of the intriguing mysteries of organic evolution. Their diversification during the Silurian and early Devonian (starting some 420 million years ago) from green algal ancestors was accompanied by capabilities of biosynthesis that made new growth habits possible. In 35 million years, a brief time in an evolutionary perspective, the first plants gave rise to the major phyletic groups of the seedless plants. Why did the land plants appear at that time, and why did they follow such a course of early development? Traditional speculation about the early history of land plants has emphasised the importance of the dry land environment, and plant organs (as sporangia and roots) were seen as adaptations against desiccation (see Bower, 1908). These ideas continue in textbook discussions on the origin of land plants as well as in the scientific literature (Walter and Stadelman, 1968). Although desiccation was very probably a selection pressure that determined the evolution of some plant organs, it falls short of explaining the timing and course of early plant evolution. The dry land habitat existed billions of years before the appearance of vascular plants in the fossil record. The green algae, which almost certainly included the progenitors of land plants (see Stewart and Mattox, 1975), existed perhaps a billion years before (Schopf and Oehler, 1976). Nor does the challenge of a dry land environment help us understand the patterns of early evolutionary development; we must look elsewhere for an approach towards answering these questions. We see two alternatives. The first is that, even with the presence of suitable algal progenitors and environments, plants appeared on land only after a delay involving the accumulation of the necessary genetic information (transcribed as biochemical capability and morphological expression). Niklas (1976) has argued that the major bottleneck delaying the arrival of land plants was their ability to synthesise lignin. He envisaged the ensuing explosive evolution of land plants in terms of the fold catas