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.