to their fused, stable structures. Although hopanoids and sterols are reduced to hopanes and steranes during deposition, these diagenetic products can still be useful biomarkers, or molecular fossils, for studying the coevolution of early life and Earth. Currently, the oldest detected undisputed triterpenoid fossils are Mesoproterozoic okenanes, steranes, and methylhopanes from a 1.64 Ga (billion year) old basin in Australia. However, molecular clock analyses estimate that the earliest sterols were likely produced around 2.3 Ga ago, around the same time as the Great Oxidation Event, with hopanoid synthesis arising even earlier. For several reasons, hopanoids and squalene-hopene cyclases have been hypothesized to be more ancient than sterols and oxidosqualene cyclases. First, diplopterol is synthesized when water quenches the C22 carbocation formed during polycyclization. This indicates that hopanoids can be made without molecular oxygen and could have served as a sterol surrogate before the atmosphere accumulated oxygen, which reacts with squalene in a reaction catalyzed by squalene monooxygenase during sterol biosynthesis. Furthermore, squalene binds to squalene-hopene cyclases in a low-energy, all-chair conformation while oxidosqualene is cyclized in a more strained, chair-boat-chair-boat conformation. Squalene-hopene cyclases also display more substrate promiscuity in that they cyclize oxidosqualene in vitro, causing some scientists to hypothesize that they are evolutionary predecessors to oxidosqualene cyclases. Other scientists have proposed that squalene-hopene and oxidosqualene cyclases diverged from a common ancestor, a putative bacterial cyclase that would have made a tricyclic malabaricanoid or tetracyclic dammaranoid product. 2-methylhopanoids As a biomarker for cyanobacteria Proposal 2-methylhopanes, often quantified as the 2-α-methylhopane index, were first proposed as a biomarker for oxygenic photosynthesis by Roger Summons and colleagues following the discovery of the precursor lipids, 2-methylhopanols, in cyanobacterial cultures and mats. The subsequent discovery of 2-α-methylhopanes supposedly from photosynthetic cyanobacteria in 2.7 Ga old shales from the Pilbara Craton of Western Australia suggested a 400 Ma (million year) gap between the evolution of oxygenic metabolism and the Great Oxidation Event. However, these findings were later rejected due to potential contamination by modern hydrocarbons. Putative cyanobacterial presence on the basis of abundant 2-methylhopanes has been used to explain black shale deposition during Aptian and Cenomanian–Turonian Ocean Anoxic Events (OAEs) and the associated 15N isotopic signatures indicative of N2-fixation. In contrast, 2-α-methylhopane index values are relatively low across similar Frasnian and Famennian sediments corresponding to the Kellwasser event(s), though higher levels have been reported in later Lower Famennian sections. Dispute The status of 2-methylhopanoids as