Chemical signals for early evolution of major taxa: Biosignatures and taxon-specific biomarkers

Chemical biosignatures may consist of biomarkers or nonrandom mixtures of lipids or other compounds that could not have been assembled by abiogenetic processes. The most obvious of these are repeating structural subunits, reflecting the biosynthetic assemblage of lipids. Biosignatures may eventually be a critical component in recognizing extraterrestrial life. Taxon-specific biomarkers (TSBs) are recognized as complex biosynthetic molecules (biomarkers) that are utilized or synthesized by one specific group of organisms. Thus, they are signature compounds with demonstrated efficacy for tracing evolutionary history and the early development of the Earth's biosphere. Examples of TSBs and how they are used are exemplified by steranes (earliest eukaryotes), 8-methylhopanes (oxygenetic photosynthesis), 24-n-propylcholestanes (chrysophyte algae), triaromatic and saturate dinosteroid hydrocarbons (dinoflagellates), triaromatic 23,24-dimethylcholesteroids (dinoflagellates, haptophytes). 24-isopropylcholestanes (porifera), 24-norcholestanes (diatoms), and oleananes (angiosperms). Occurrence of TSBs before the recognized appearance of the taxa they represent is viewed as a new opportunity to obtain an early record of evolution. When applied to major taxa, important information about Earth's ancient environments can be determined.