The evolution of Earth’s biota is intimately linked to the oxygenation
of the oceans and atmosphere. We use the isotopic composition
and concentration of molybdenum (Mo) in sedimentary rocks
to explore this relationship. Our results indicate two episodes of
global ocean oxygenation. The first coincides with the emergence
of the Ediacaran fauna, including large, motile bilaterian animals,
ca. 550–560 million year ago (Ma), reinforcing previous geochemical
indications that Earth surface oxygenation facilitated this
radiation. The second, perhaps larger, oxygenation took place
around 400 Ma, well after the initial rise of animals and, therefore,
suggesting that early metazoans evolved in a relatively low oxygen
environment. This later oxygenation correlates with the diversification
of vascular plants, which likely contributed to increased
oxygenation through the enhanced burial of organic carbon in
sediments. It also correlates with a pronounced radiation of large
predatory fish, animals with high oxygen demand. We thereby
couple the redox history of the atmosphere and oceans to major
events in animal evolution.