Hopanes and steranes are both abundant in sediments because their biological precursors are important components of cell membranes of prokaryotes and eukaryotes. It appears, however, that both of these compounds are derived from the same precursor, squalene. Though what is shown below is vastly oversimplified, it gives you an idea how the structure of squalene, if cyclized using the right enzymes, and under different redox conditions, can form either sterols (below, right) or hopanoids (below, left).


Because oxygen is required during biosynthesis of sterols, but not for bacteriohpanetetrol, which is the hopanoid in prokaryotic cell membranes, it is hypothesized that the biosynthetic pathway to sterols evolved after oxygen was present on the early earth.

Both hopanes and steranes are types of polycyclic triterpanes.