The long infancy of sterol biosynthesis

Brocks and colleagues also clearly show that later, around 800 million years ago, during the Tonian period (which spanned 1,000 million years ago to 720 million years ago), the oldest-known molecular fossils of crown sterols (the aromatic hydrocarbons of cholesterol and ergosterol) replaced fossil protosterols as the most abundant fossil sterols. Brocks and colleagues' data-gathering approach was unusual and creative because aromatic steroids were used for the first time to establish a long-term mid-Proterozoic record of fossil steroids (molecules that have sterol precursors). Another argument weakening the proposal of a bacterial origin for these protosteroids is the occurrence of 4,24-dimethyl triaromatic steroid around 1,300 million years ago, the potential precursors of which have not been observed in bacterial extracts.