The Neoproterozoic Era is of widespread geobiological interest because it marks the critical transition from a world of microbes to one where animals become an established feature of the landscape. Much research into this time period has focused on the ventilation of the oceans, as this is widely considered a primary factor driving the diversification of complex, multicellular life. In this study, Proterozoic to Cambrian aged oils from eastern Siberia were analyzed for their hydrocarbon biomarker contents and compound specific carbon isotopes in order to further our understanding of the prevailing environment and its microbial and metazoan communities. Geochemically, these oils are broadly comparable to those of the Ediacaran–Cambrian sedimentary rocks and oils of the South Oman Salt Basin. Organic matter in the source sedimentary rocks included significant contributions from green algae, demosponges and bacteria including cyanobacteria and methanotrophic proteobacteria. Although the ages of the Siberian oils and putative parent source rock intervals are poorly constrained, the geochemical similarities between the Ediacaran Oman Huqf and Nepa-Botuoba-Katanga family of Siberian oil samples are impressive, leading to the inference that their source rocks are coeval. On the other hand, oils from the Baykit High are distinctive, likely older and possibly of Cryogenian age.

The Neoproterozoic era (1,000–542 Myr ago) was an era of climatic extremes and biological evolutionary developments culminating in the emergence of animals (Metazoa) and new ecosystems1. Here we show that abundant sedimentary 24-isopropylcholestanes, the hydrocarbon remains of C30 sterols produced by marine demosponges, record the presence of Metazoa in the geological record before the end of the Marinoan glaciation (~635 Myr ago). These sterane biomarkers are abundant in all formations of the Huqf Supergroup, South Oman Salt Basin, and, based on a new high-precision geochronology2, constitute a continuous 100-Myr-long chemical fossil record of demosponges through the terminal Neoproterozoic and into the Early Cambrian epoch. The demosponge steranes occur in strata that underlie the Marinoan cap carbonate (>635 Myr ago). They currently represent the oldest evidence for animals in the fossil record, and are evidence for animals pre-dating the termination of the Marinoan glaciation. This suggests that shallow shelf waters in some late Cryogenian ocean basins (>635 Myr ago) contained dissolved oxygen in concentrations sufficient to support basal metazoan life at least 100 Myr before the rapid diversification of bilaterians during the Cambrian explosion. Biomarker analysis has yet to reveal any convincing evidence for ancient sponges pre-dating the first globally extensive Neoproterozoic glacial episode (the Sturtian, ~713 Myr ago in Oman2).