Genetics re-establish the utility of 2-methylhopanes as cyanobacterial biomarkers before 750 million years ago

Abstract Fossilized lipids offer one of only few windows into ancient ecosystems. The utility of such biomarkers is determined by the phylogenetic distribution of lipid biosynthetic capabilities in extant organisms and extrapolation of this information into the past. 2-Methylhopanes in sedimentary rocks were once used to infer the importance of Cyanobacteria as primary producers in the geological past. However, the discovery of the hopanoid C-2 methyltransferase (HpnP) in Alphaproteobacteria led to the downfall of this molecular proxy. In the present study, we re-examined the distribution of HpnP in a new phylogenetic framework including novel candidate phyla, and re-interpreted a revised geological record of 2-methylhopanes based on contamination-free samples. We show that HpnP was likely present in the last common ancestor of Cyanobacteria, while the gene appeared in Alphaproteobacteria only around 750 million years ago. A subsequent rise of sedimentary 2-methylhopanes around 600 Ma likely reflects the expansion of Alphaproteobacteria that coincided with the rise of eukaryotic algae — possibly connected by algal dependency on microbially-produced vitamin B12. Our findings re-establish 2-methylhopanes as cyanobacterial biomarkers prior to 750 Ma and thus as a potential tool to measure the importance of oxygenic Cyanobacteria as primary producers on the early Earth. Our study illustrates how genetics can improve the diagnostic value of biomarkers and refine the reconstruction of early ecosystems.