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The sudden appearance of complex animals in the Cambrian period puzzled Darwin. He regarded it as one of the most important problems to beset his theory of evolution by natural selection. Here, Jochen Brocks and colleagues show that the Cambrian ‘explosion’ was preceded by a ‘rise of algae’ during an interval in which the world may have been largely frozen over. Various steroids preserved in sediments are distinctive markers of eukaryotes, but steroids typical of algae only abound for a short interval in the Cryogenian period between the Sturtian (720–660 Ma) and Marinoan (650–635 Ma) glaciations. In this relatively short, warm interval, phosphorus released by Sturtian weathering allowed eukaryotes to flourish. This broke the stranglehold on Earth's ecology by cyanobacteria, which can get by in lower phosphorus concentrations. This ‘rise of algae’ created shorter, more efficient food webs, driving an escalatory race towards larger and increasingly complex organisms and the rise of animals.

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