The Permian-Triassic Extinction Event, or the Great Dying, was the single largest extinction event in Earth's history. Around 252 million BC, up to 90% of Earth's lifeforms disappeared from the fossil record.
This event fell into a period of galactic history between roughly 250 and 100 million BC, during which no evidence of intelligent life existed. This mystery remained unsolved until 2358, when it was revealed that the Manraloth had tried to forcibly "ascend" all intelligent life to a higher plane of existence. (TNG - The Lost Era novel: The Buried Age)