Tethys went down with a chunk of Earth's crust 180 million years ago. When Gondwana broke apart, the crust slipped beneath the Eurasian plate. When this happened, shattered fragments of the crust ...
About 800 million years ago, oxygen levels reached about 21 percent and began to breathe life into more complex organisms. The oxygen-rich ozone layer was also established, shielding the Earth's ...
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