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.陆地定殖过程要求生物演化出全新策略，以保障配子与传播体（diaspore）的安全扩散，并应对脱水、有害辐射、火灾及重力胁迫。相应地，生物体的形态、行为与生理机能均发生了适应性改变。本文旨在探讨：保存在沉积物分子组分中的生理适应特征，在多大程度上能够加深我们对陆生演化（terrestrialization）的认知。许多被视为陆地植物标志性特征的化合物，无法从化石记录中获取有效信息，原因如下：其一，该类化合物未得以保存；其二，其存在或对应物质早在陆生演化之前就已演化形成（例如角质膜（cutan）与藻角质（algaenan）、纤维素（cellulose））；其三，此类化合物经历了成岩作用（diagenesis）和/或后生变质作用（catagenesis）的改造。上述第三种情形会导致地质（大）分子（geo(macro)molecules）无法通过化学指纹匹配其原始生物（大）分子（bio(macro)molecules），即便有机残骸有时仍保留着极佳的形态结构。尽管如此，部分分子标记物及其稳定同位素仍可独立提供关于陆生演化过程的相关信息。正构烷烃（n-alkane）表层蜡质的异常富集现象，早在早期陆地植物中就已显现，经谨慎评估后可作为陆生演化的分子标识之一。此外，化石萜类化合物（terpenoids）及其衍生物，对于重建主要植物类群的演化历程具有重要参考价值。以孢粉素（sporopollenin）和木质素（lignin）为代表的苯丙烷代谢途径（phenylpropanoid pathway）的演化辐射，似乎与陆地植物的演化进程密切相关。