The transition from simple, single cellular eukaryotes to more complex multicellular organisms, including animals, represents one of life’s most profound evolutionary advances. Yet, the temporal dynamics and natural selection pressures which led to the evolution of animals remains enigmatic. A detailed investigation of the evolutionary history of the domain Eukaryota can offer valuable insights and an enhanced resolution of this momentous transition. This includes an examination of pivotal events such as the first recorded appearance of eukaryotes in the microfossil record during the Mesoproterozoic, versus the temporal lag associated with the rise of eukaryotes to ecological dominance during the Neoproterozoic Era as shown by lipid biomarker geochemistry. Steroid biomarkers were employed to track the evolution, expansion, diversification and the ecological rise of eukaryotes relative to bacteria in the geologic rock record.

We used multiple lipid biomarker proxies to track the evolution of the marine biosphere with carefully selected sedimentary rock targets that had undergone a mild thermal history and that were not adversely affected by organic contaminants. The findings described here include i) the oldest kerogen-bound sterane signals ever reported from thermally well-preserved Tonian rocks (ca. 780-729 Ma.), prior to the first (Sturtian) Neoproterozoic glaciation event, ii) a new C30 ancient metazoan steroid biomarker recovered from the Cryogenian-Ediacaran rock record and classification of the equivalent sterol precursors in a number of modern demosponge species and iii) a systematic investigation of the biosynthetic pathways involved in the production of conventional and unconventional sponge sterols that yield attractive ancient biomarker targets. The unique analytical approaches in this study, including the analysis of covalently bound biomarkers recovered from the kerogen-bound phase of organic matter, push the boundary of our understanding of the timing of eukaryotic diversification and expansion in the Neoproterozoic marine biosphere. For modern sponge taxa, a combination of intact sterol analyses and hydrogenation of sterols to sterane derivatives allowed for the identification of several novel C29 and C30 ancient sponge sterane biomarkers and provided a better understanding of the steroid biosynthetic pathways and product-precursor relationships.