Common origin of sterol biosynthesis points to a feeding strategy shift in Neoproterozoic animals

Steranes preserved in sedimentary rocks serve as molecular fossils, which are thought to record the expansion of eukaryote life through the Neoproterozoic Era ( ~ 1000-541 Ma). Scientists hypothesize that ancient C₂₇ steranes originated from cholesterol, the major sterol produced by living red algae and animals. Similarly, C₂₈ and C₂₉ steranes are thought to be derived from the sterols of prehistoric fungi, green algae, and other microbial eukaryotes. However, recent work on annelid worms-an advanced group of eumetazoan animals-shows that they are also capable of producing C₂₈ and C₂₉ sterols. In this paper, we explore the evolutionary history of the 24-C sterol methyltransferase (smt) gene in animals, which is required to make C₂₈₊ sterols. We find evidence that the smt gene was vertically inherited through animals, suggesting early eumetazoans were capable of C₂₈₊ sterol synthesis. Our molecular clock of the animal smt gene demonstrates that its diversification coincides with the rise of C₂₈ and C₂₉ steranes in the Neoproterozoic. This study supports the hypothesis that early eumetazoans were capable of making C₂₈₊ sterols and that many animal lineages independently abandoned its biosynthesis around the end-Neoproterozoic, coinciding with the rise of abundant eukaryotic prey.