2-methylhopanes are biosynthesised by cyanobacteria (oxygenic photosynthetic bacteria) and are found in 2.7Gya shales, they are indicators of

Evolution of oxygenic photosynthesis (produces oxygen) well before the atmosphere became oxidizing. HOWEVER 2-MeBHPs have been found in an anoxygenic phototroph! 2-methylhopanes can’t be used for biomarkers of oxygenic photosynthesis