進化：古代のステロイドが物語る先史時代の生物

A new type of steroid has been discovered in ancient rocks. The findings, reported in this week’s Nature, confirms the deep prehistory of complex eukaryotic life and fulfils the prediction of Nobel Prize winner Konrad Bloch, who posited the existence of primordial steroid molecules.

Eukaryotes are thought to have been in existence for as long as 2 billion years. Fossils of these organisms are scarce, so researchers have instead tried searching for molecular fossils — such as distinct steroid metabolites that eukaryotes may leave behind in rocks. However, these fossils have also been difficult to find. Jochen Brocks and colleagues suggest that these biomarkers were there all along, albeit in a different form than was expected. They report the discovery of molecules known as protosteroids in mid-Proterozoic sedimentary rocks, confirming the ecological dominance of early eukaryotes in aquatic environments from at least 1.6 to 0.8 billion years ago.

Such molecules were first predicted to have existed by Konrad Bloch in 1994. He proposed that each short-lived intermediate in the long biosynthetic pathway from lanosterol to cholesterol was once, hundreds of millions of years ago, a fully adapted end-product and that each of these products was an evolutionary improvement over its precursor. He concluded, however, that these intermediate primordial molecules would never be found because they cannot survive in the geological record. The research from Brocks and colleagues indicates that this is not the case, and suggests that traces of protosteroids can persist in rock for more than a billion years.