The dawn of animals remains one of the most mysterious milestones in the evolution of life. The fossil lipids 24-isopropylcholestane and 26-methylstigmastane are considered diagnostic for demosponges—arguably the oldest group of living animals. The widespread occurrence and high relative abundance of these biomarkers in Ediacaran sediments from 635–541 million years (Myr) ago have been viewed as evidence for the rise of animals to ecological importance approximately 100 Myr before their rapid Cambrian radiation. Here we show that the biosynthesis of 24-isopropylcholestane and 26-methylstigmastane precursors is common among early-branching unicellular Rhizaria—heterotrophic protists that play an important role in trophic cycling and carbon export in the modern ocean. Negating these hydrocarbons as sponge biomarkers, our study places the oldest evidence for animals closer to the Cambrian Explosion. Cambrian silica hexactine spicules that are approximately 535 Myr old now represent the oldest diagnostic sponge remains, whereas approximately 558-Myr-old Dickinsonia and Kimberella (Ediacara biota) provide the most reliable evidence for the emergence of animals. The proliferation of predatory protists may have been responsible for much of the ecological changes during the late Neoproterozoic, including the rise of algae, the establishment of complex trophic relationships and the oxygenation of shallow-water habitats required for the subsequent ascent of macroscopic animals.
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The data required to assess the interpretations made in this paper are included in the Supplementary Information. Additional (raw) data are available from the corresponding authors upon reasonable request.
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We thank P. Pringle and R. Tarozo for laboratory support; A. Leider, Y. Hoshino, M. Neumann, N. Kuznetsov and L. van Maldegem for discussions and reference samples; M. Holzmann, R. Sierra, J. Bernhard, S. Eggins, C. Bachy and C. Reymond for assistance in sourcing specimens; and S. Porter, R. Meisterfeld, S. Pruss, S. Chang and J. Botting for fossil images. This study was principally funded by the Max Planck Society (to C.H. and R.S.) and the Agouron Institute (Geobiology fellowship to B.J.N.). We also acknowledge the US National Science Foundation (grant nos. PLR134161 to S.S.B. and DBI-1349350 to M.W.L.), Swiss National Science Foundation (grant no. 31003A_179125 to J.P.), the German Research Foundation (grant no. NO1090/1-1 to E.N.), the Leibniz Association (grant no. SAW-2014-ISAS-2 to M.S.), Formas, the Swedish Research Council (A.S.), the French National Research Agency (grant no. IMPEKAB ANR-15-CE02-001 to F.N.) and Australian Research Council (grant nos. DP1095247 and DP160100607 to J.J.B.).