Scientists at the University of Western Australia have proposed an explanation for the emergence of Earth's oxygen-rich atmosphere and the evolution of life. The results of the study are published in the journal Nature.

The first major event to affect animal evolution took place during the Ediacaran period between 570 and 550 million years ago and is thought to be a massive release of carbon dioxide and oxygen into the atmosphere and oceans as a result of an increase in phosphorus in seawater. To confirm this hypothesis, scientists tracked the phosphorus content in ocean sediments in Australia, China, Mexico and the United States.

Phosphorus is considered to be the main nutrient that limits the productivity of the marine environment over geologic time scales. Phosphorus dissolved in sea water enhances photosynthesis, which results in the production of oxygen. This prevents global ocean anoxia and also supplies oxygen to eukaryotic life forms, allowing them to evolve over millions of years.

However, it turned out that the increase in the level of phosphorus in the ocean cannot explain the increase in the oxygen content in the atmosphere. During times of global ocean oxygenation, phosphorus levels rose in discrete pulses. This can be explained by the release of carbon dioxide and phosphorus from the oxidation of marine organic matter, mainly sulfates, followed by the release of phosphorus from onshore weathering caused by carbon dioxide.

The result indicates that extrinsic stimuli, such as sulfate weathering, played a role in the oxygenation of the oceans on Earth, and not just the internal cycle of phosphorus and oxygen in the ocean. In turn, this could help explain the long-term rise in atmospheric oxygen levels.