An organic geochemical perspective on terrestrialization

The colonization of land required new strategies for safe gamete/diaspore dispersal, and to cope with desiccation, harmful
radiation, fire and gravity. Accordingly, the morphology, behaviour and physiology of the organisms changed. Here, we explore
to what extent physiological adaptations, reflected in the molecular content of the sediments, add to our understanding of
the terrestrialization. Many compounds considered characteristic of land organisms do not provide valuable information from
the fossil record since (1) they were not preserved; (2) they occur or correspond to substances that evolved prior to the
terrestrialization (e.g. cutan vs. algaenan, cellulose); or (3) they have been changed diagenetically and/or catagenetically.
The latter leads to geo(macro)molecules without a chemical fingerprint relating them to their original bio(macro)molecules
despite, sometimes, excellent morphological preservation of the organic remains. Nevertheless, some molecular markers and
their stable isotopes provide independent information on the terrestrialization process. The odd predominance of n-alkane surface waxes is a feature already apparent in early land plants and could, with caution, be used as such. Furthermore,
fossil terpenoids and their derivatives are valuable for reconstructing the evolution of major plant groups. The radiation
of the phenylpropanoid pathway with for example, sporopollenin and lignin seems to be closely related to the evolution of
land plants.