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要旨

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.