Answer
The causes of the known major mass extinctions are not all known, but they generally relate to a global change in the atmosphere and climate so great that a large portion of all species at the time could not adapt readily enough, causing many species to die off.
Work Step by Step
Recovery can take anywhere from 5 to 30 million years, depending on the cause and extent of the damage done. At a minimum, this is dramatically longer than humans have even existed.
