Bisnorgammacerane traces predatory pressure and the persistent rise of algal ecosystems after Snowball Earth.

Eukaryoticalgae rose to ecological relevance after the Neoproterozoic Snowball Earthglaciations, but the causes for this consequentialevolutionary transition remain enigmatic. Cap carbonateswere globally deposited directly after these glaciations, but they are usually organic barren or thermally overprinted. Here we show that uniquely-preserved cap dolostonesof the Araras Group contain exceptional abundances of a newly identified biomarker: 25,28-bisnorgammacerane. Its secular occurrence, carbon isotope systematics and co-occurrence with other demethylated terpenoidssuggest a mechanistic connection to extensive microbial degradation of ciliate-derived biomass in bacterially dominated ecosystems. Declining 25,28-bisnorgammacerane concentrations, and a parallel rise of steranesover hopanes, indicate the transition from a bacterial to eukaryoticdominated ecosystem after the Marinoan deglaciation. Nutrient levels already increased during the Cryogenian and were a prerequisite, but not the ultimate driver for the algal rise. Intense predatory pressure by bacterivorous protistsmay have irrevocably cleared self-sustaining cyanobacterial ecosystems, thereby creating the ecological opportunity that allowed for the persistent rise of eukaryoticalgae to global importance.