Paleoproterozoic sterol biosynthesis and the rise of oxygen

Steranes in ancient rocks have been used as ‘molecular fossils’, but the very earliest records of steranes have been shown to be contaminants; here, the presence of two key sterol biosynthesis enzymes in eukaryotes and bacteria suggests at least one gene transfer between bacteria and the earliest eukaryotes occurred some 2.3 billion years ago. The spread of sterol biosynthesis Hydrocarbons called steranes are sometimes found in ancient rocks. They can be viewed as 'molecular fossils' of sterol biosynthesis, which is usually associated both with eukaryotes and with abundant oxygen, because so much is required. However, the earliest records of steranes have been shown to be contaminants. Here the authors look at the presence of two key sterol biosynthesis enzymes in eukaryotes and bacteria. They suggest that gene transfer between bacteria and the earliest eukaryotes occurred at least once, some 2.3 billion years ago, coincident with the Great Oxidation Event. Natural products preserved in the geological record can function as ‘molecular fossils’, providing insight into organisms and physiologies that existed in the deep past. One important group of molecular fossils is the steroidal hydrocarbons (steranes), which are the diagenetic remains of sterol lipids. Complex sterols with modified side chains are unique to eukaryotes, although simpler sterols can also be synthesized by a few bacteria 1 . Sterol biosynthesis is an oxygen-intensive process; thus, the presence of complex steranes in ancient rocks not only signals the presence of eukaryotes, but also aerobic metabolic processes 2 . In 1999, steranes were reported in 2.7 billion year (Gyr)-old rocks from the Pilbara Craton in Australia 3 , suggesting a long delay between photosynthetic oxygen production and its accumulation in the atmosphere (also known as the Great Oxidation Event) 2.45–2.32 Gyr ago 4 . However, the recent reappraisal and rejection of these steranes as contaminants 5 pushes the oldest reported steranes forward to around 1.64 Gyr ago (ref. 6 ). Here we use a molecular clock approach to improve constraints on the evolution of sterol biosynthesis. We infer that stem eukaryotes shared functionally modern sterol biosynthesis genes with bacteria via horizontal gene transfer. Comparing multiple molecular clock analyses, we find that the maximum marginal probability for the divergence time of bacterial and eukaryal sterol biosynthesis genes is around 2.31 Gyr ago, concurrent with the most recent g