Selenium isotopes record extensive marine suboxia during the Great Oxidation Event

Significance Oxygen is essential for eukaryotic life. The geologic record of early Earth contains abundant evidence of low oxygen levels, and accordingly, a lack of eukaryote fossils. The rise of oxygen to near-modern levels at the end of the Proterozoic Era is thus often cited as the trigger for the evolutionary radiation of complex life forms at this same time. Here we present selenium geochemical data that indicate an expansion of suboxic (>0.4 μM O 2 ) habitats in the shallow oceans between 2.32 and 2.1 Ga––more than one billion years before eukaryotes become abundant in the fossil record. These environments could have harbored the earliest stages of eukaryotic evolution, but may have been too transient for substantial diversification to occur.