In his 1994 essay “Evolutionary perfection of a small molecule,” Konrad Bloch predicted that primordial eukaryotes may not have possessed a full sterol biosynthetic pathway but instead generated ancestral “protosterols” and “ursterols.” He suggested that each of these intermediates in the long biosynthetic pathway toward cholesterol was, hundreds of millions of years ago, a fully functional membrane lipid, and that each additional biosynthetic modification was a Darwinian improvement over its precursors. At some point, deep in geological time, each “ursterol” must thus have been a functional end-product in the membrane of an ancestral eukaryote that preceded the last eukaryotic common ancestor (LECA). Bloch’s hypothesis predicts that the geological record may hide an array of fossil sterols that record the sequence of assembly of the original biosynthetic pathway in deep time. While Konrad Bloch did not believe that such ancient sterols could be preserved, we report the discovery of protosteroids up to 1.6 billion years old (Ga). The fossil hydrocarbons were solvent-extracted from sedimentary rocks that formed the seafloor of marine basins in the Proterozoic Eon. The protosteroids reveal an ecologically prominent “Protosterol Biota” that was widespread and abundant in aquatic environments between ∼1.6 and 0.8 Ga ago and that likely comprised ancient protosterol-producing bacteria and deep-branching stem-group eukaryotes. The primordial eukaryotes apparently declined in the Tonian period (1.0 to 0.72 Ga), freeing ecospace for the expansion of algae and other modern eukaryotes that possessed a complete sterol biosynthetic pathway. This “Tonian Transformation” emerges as one of the most profound ecological turning points in our planet’s history.
Brocks et al. (Sun,) studied this question.
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