Originally posted by sonhouse
I can do left! Left is good!
The answer is 66 million years. I thought it was 65 million, but it appears they've pushed it back another million since I was in school.
This resulted in the K-T event (which they now call the K-Pg event, grrrrrr) which separates the Cretaceous period from the Paleogene period. A layer of the rare element iridium (as well as some osmium) defines the geological layer laid down after the impact.
This comet hypothesis is new also. More research needs to be done. For years it was thought to be an asteroid, of the Baptistina family of asteroids, but recently it's been proposed that it was a ~10km sized comet (long period type) traveling ~32 km/s.
Yes, widespread devastation. Or as they say in Texas, that there's some good old-fashioned Texas barbecue. A climate similar to a nuclear winter (brought on by the impact) probably helped more species along the road to extinction even more so than the actual impact.
The obvious answer to your last question is because this occurred before man appeared on the planet.