The Permian–Triassic (P–Tr) extinction event, informally known as the Great Dying, was an extinction event that occurred 251.4 million years ago, forming the boundary between the Permian and Triassic geologic periods. It was the Earth's most severe extinction event, with up to 96 percent of all marine species and 70 percent of terrestrial vertebrate species becoming extinct; it is the only known mass extinction of insects. 57% of all families and 83% of all genera were killed off. Because so much biodiversity was lost, the recovery of life on earth took significantly longer than after other extinction events. This event has been described as the "mother of all mass extinctions". The pattern of extinction is still disputed, as different studies suggest one to three different pulses. There are several proposed mechanisms for the extinctions; the earlier peak was likely due to gradualistic environmental change, while the latter was probably due to a catastrophic event. Possible mechanisms for the latter include large or multiple bolide impact events, increased volcanism, or sudden release of methane hydrates from the sea floor; gradual changes include sea-level change, anoxia, increasing aridity, and a shift in ocean circulation driven by climate change.
This allowed the Dinosaurs to evolve.