Sterol Methyltransferases in Annelid Worms Rewrite the Molecular Fossil Record

Abstract

Molecular fossils have been used to augment the physical fossil record, charting the expansion of complex life through the Neoproterozoic Era (~1000-541 Ma). This work relies on the hypothesis that C 27 steranes preserved in sedimentary rocks originated from cholesterol, the predominant sterol produced by red algae and animals. Following the same logic, C 28 and C 29 carbon steranes are widely considered to be derived from the sterols of fungi, green algae and some protists. In this study, we demonstrate that the gene 24-C sterol methyltransferase (smt), which is necessary to produce C 28 and C 29 sterols, exists in segmented worms, an advanced group of animals. Phylogenetic analysis of the relevant gene suggests it was present in the first animals and lost independently in at least seven major lineages. A molecular clock demonstrates that the SMT specific to animals and their closest ancestors was present through the Neoproterozoic. Based on these results, C 27 steranes cannot be considered indicative of Neoproterozoic animals, and C 28+ steranes are not solely indicative of fungi or green algae. While our results do not necessarily contradict the emerging picture of Neoproterozoic life informed by molecular fossils, they refute the underlying hypothesis that drives the interpretive paradigm.