Transcript: The best evidence we have that mass extinction can be caused by an impact was the extinction that occurred sixty-five million years ago at the boundary between the Cretaceous and Tertiary periods, the so called KT impact. An impact directly killed forty percent of all plants and animals and ultimately through its effect on the atmosphere and global cooling, seventy-five percent of all plants and animals. Possibly ninety-nine percent of all the organisms were killed in this event. Among the few survivors were mammals, which of course were our descendants. The evidence that this mass extinction was caused by an impact is very good. There is crater of exactly the right age off the Yucatan Peninsula. The fossil record shows and almost instantaneous loss of species, and there is widespread or global evidence for shocked quartz, for soot layers corresponding to fires that were initiated, and for deposition of iridium which is material that only comes from outer solar system material.