Sedimentary Records of Biomarkers as Milestones for Evolutionary Innovations

Compilations of the geological occurrences of biomarkers that derive from specific source organisms can provide a historical legacy of evolutionary changes in Life through time. The nature of the molecular and isotopic records accessible in the geological past present significant challenges in efforts to unravel their testimony of biosynthetic innovations, including: (i) the innate conservatism of key biosynthetic pathways (e.g. carboxylic acids) which results in similarities in dominant components, notably membrane lipids, across broad domains, (ii) the inherent gaps in knowledge of extant biosynthetic pathways and the breadth of occurrences of diagnostic lipids that require revision as new evidence emerges (e.g. 4-methylsterols in diatoms), (iii) the intrinsic environmental variables, especially temperature, salinity, and nutrient availability, that control biolipid production (e.g. wax esters, 2-methylhopanoids), which precludes attribution of major significance to the absence of components, (iv) the initial scarcity of components that are integral to evolutionary change (e.g. oleanoids) and the difficulty in determining precise timing for their origination from such low abundances, and (v) the inevitable variability in the survival of key structural attributes (e.g. positions and stereochemistry of methylation) and diagnostic components (e.g. 7-, 8-methylheptadecanes from cyanobacteria) during diagenetic alteration.

Irrespective of these constraints various compilations of biomarker occurrences throughout the geological record (i) provide consistent evidence for the persistence of ancient biosynthetic pathways, notably production of phytol for chlorophylls, and (ii) indicate the likely advent of new traits linked to evolutionary developments in photosynthetic organisms based on the first appearance of specific components, such as isorenieratane from green sulfur bacteria, alkenones from selected haptophytes and botryococcane from a single species of green alga.