MOLECULAR CLOCKS FOR MOLECULES: AN INTERSECTION OF GEOBIOLOGY AND GEOCHRONOLOGY

Sterols are essential constituents of eukaryotic membranes and there is an absolute requirement for molecular oxygen at many points during their biosynthesis. Thus, fossil steroids are potentially important for the clues they can provide about the history of aerobiosis and eukaryotes especially in the absence of diagnostic features for these processes in the fossil record. In the particular case of the sponge biomarker hypothesis, the overt prevalence of 24-isopropylcholestane in rocks ~650-540 million years old has been attributed to sponges, a proposition supported by the occurrence of a precursor sterol, 24-isopropylcholesterol or 24-ipc, in some modern demosponges. Arguments against the sponge biomarker hypothesis rest on the prevalence of a structurally similar sterol in Pelagophyte algae. With the aid of a phylogenetic tree for a key enzyme in the biosynthesis 24-ipc, and using a molecular clock approach calibrated to the first appearances of specific eukaryote fossils, it was recently shown that a gene responsible for 24-ipc production in sponges arose in a gene duplication event that overlapped with the appearance of 24-isopropylcholestanes in Neoproterozoic sediments (Gold et al., 2016). In contrast, the gene that performs the analogous function in Pelagophyte algae arose independently much later and during the Paleozoic. In the present study, we report how a similar approach can be used to find the temporal origin of genes that code for the first two proteins in the sterol biosynthesis pathway. We found that two genes, squalene monooxygenase (SQMO) and oxidosqualene cyclase (OSC) most likely originated around 2.33Ga and synchronously with the Great Oxidation Event as identified by the disappearance of non-mass-dependent fractionation of sulfur isotopes in sedimentary pyrite (Luo et al., 2016). All of these observations rest on foundational improvements in the accuracy and precision in the dating of sedimentary rocks.

Gold DA et al. (2016) Sterol and genomic analyses validate the sponge biomarker hypothesis. PNAS 113, 2684-2689.

Luo G et al. (2016) Rapid oxidation of Earth’s atmosphere 2.33 billion years ago. Science Advances 2, e1600134.