The rise of algae in Cryogenian oceans and the emergence of animals

Steroid biomarkers provide evidence for a rapid rise of marine planktonic algae between 659 and 645 million years ago, establishing more efficient energy transfers and driving ecosystems towards larger and increasingly complex organisms. When algae bloomed 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. The transition from dominant bacterial to eukaryotic marine primary productivity was one of the most profound ecological revolutions in the Earth’s history, reorganizing the distribution of carbon and nutrients in the water column and increasing energy flow to higher trophic levels. But the causes and geological timing of this transition, as well as possible links with rising atmospheric oxygen levels 1 and the evolution of animals 2 , remain obscure. Here we present a molecular fossil record of eukaryotic steroids demonstrating that bacteria were the only notable primary producers in the oceans before the Cryogenian period (720–635 million years ago). Increasing steroid diversity and abundance marks the rapid rise of marine planktonic algae (Archaeplastida) in the narrow time interval between the Sturtian and Marinoan ‘snowball Earth’ glaciations, 659–645 million years ago. We propose that the incumbency of cyanobacteria was broken by a surge of nutrients supplied by the Sturtian deglaciation 3 . The ‘Rise of Algae’ created food webs with more efficient nutrient and energy transfers 4 , driving ecosystems towards larger and increasingly complex organisms. This effect is recorded by the concomitant appearance of biomar