Algal origin of sponge sterane biomarkers negates the oldest evidence for animals in the rock record

The earliest fossils of animal-like organisms occur in Ediacaran rocks that are approximately 571 million years old. Yet 24-isopropylcholestanes and other C 30 fossil sterol molecules have been suggested to reflect an important ecological role of demosponges as the first abundant animals by the end of the Cryogenian period (>635 million years ago). Here, we demonstrate that C 30 24-isopropylcholestane is not diagnostic for sponges and probably formed in Neoproterozoic sediments through the geological methylation of C 29 sterols of chlorophyte algae, the dominant eukaryotes at that time. These findings reconcile biomarker evidence with the geological record and revert the oldest evidence for animals back into the latest Ediacaran. By subjecting chlorophyte lipid extracts to pyrolysis, the authors demonstrate that the lipid biomarkers 24-isopropylcholestane and 24- n -propylcholestane can be generated from algal C 29 sterol in experiments simulating diagenetic processes, thereby undermining their status as sponge biomarkers.