EarthÕs most recent major ecological catastrophe...
...occurred 65 million years ago.
The fossil record changes abruptly before and after this moment.  It indicates that the dinosaurs disappeared at this time.  In addition, the majority of other ocean and land animals went extinct.
What happened?  The evidence supports the idea that the earth was struck by an asteroid.  Asteroids are constantly colliding with planetary bodies, as evidenced by craters on their surfaces.