Final answer:
Over the past 500 million years, five mass extinction events, which led to the loss of over 50 percent of species at those times, have been identified through the fossil record.
Step-by-step explanation:
There have been five mass extinctions events documented in the fossil record over the past 500 million years. These catastrophic events are marked by the sudden loss of biodiversity, with more than 50 percent of extant species disappearing. The five mass extinction events occurred approximately 450 million years ago, 360 million years ago, 250 million years ago, 205 million years ago, and 65 million years ago, the latter believed to be caused by an asteroid impact. In addition to these five major extinctions, there is ongoing discussion about a potential sixth extinction linked to human activity.