2-METHYLHOPANES AS UNIQUE CYANOBACTERIAL BIOMARKERS BEFORE 1.2 BILLION YEARS AGO

Summary The bioavailability of molecular oxygen is considered a driving force for the evolution of phenotypic complexity, yet the geological history of biological oxygen production remains poorly constrained. Fossilized 2-methylhopanoids (2-methylhopanes) were once considered diagnostic for the presence of cyanobacteria and oxygenic photosynthesis. However, the subsequent discovery of C-2 hopanoid methyltransferase (hpnP) in non-cyanobacterial species questioned the utility of 2-methylhopanes as cyanobacterial biomarkers. Here we re-examined the distribution of HpnP in the bacterial domain and the evolutionary history of HpnP. Our results suggest that HpnP did not originate within α-proteobacteria but rather was already present in the common ancestor of cyanobacteria, contrary to previous inferences. Molecular clock analyses further suggest that the emergence of HpnP in cyanobacteria substantially predates the emergence of HpnP in α-proteobacteria and 2-methylhopanes in rocks deposited prior to 1.2 Ga are inferred to be derived uniquely from cyanobacteria. Our results imply that 2-methylhopanes can theoretically allow us to trace the rise of oxygenic photosynthesis even prior to the GOE if suitable sedimentary sequences or preservation windows should be found in the future.