The cause of the largest extinction in Earth’s history has been discovered

Such significant changes have rarely been observed in rocks less than two billion years old because of the gradual increase in the planet’s oxygen supply and ozone formation over time. Thus, scientists hypothesize that photolysis occurs in the absence or significant depletion of the ozone layer.

Once explosive volcanism destroyed the ozone layer, there were fewer oxygen molecules to block incoming solar ultraviolet radiation, leading to the conversion of sulfur dioxide molecules into sulfate aerosols that were transported from land to the oceans. This is supported by modeling that suggests a decline in atmospheric oxygen from 30 percent in the Phanerozoic (which began 538 million years ago) to half that amount by the end of the Permian.