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 C(27) steranes originated from cholesterol, the major sterol produced by living red algae and animals. Similarly, C(28) and C(29) 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 C(28) and C(29) sterols. In this paper, we explore the evolutionary history of the 24-C sterol methyltransferase (smt) gene in animals, which is required to make C(28+) sterols. We find evidence that the smt gene was vertically inherited through animals, suggesting early eumetazoans were capable of C(28+) sterol synthesis. Our molecular clock of the animal smt gene demonstrates that its diversification coincides with the rise of C(28) and C(29) steranes in the Neoproterozoic. This study supports the hypothesis that early eumetazoans were capable of making C(28+) sterols and that many animal lineages independently abandoned its biosynthesis around the end-Neoproterozoic, coinciding with the rise of abundant eukaryotic prey.