Changes

===Extinction===

===Extinction===

[[Image:800px-KT_boundary_054.jpg|thumb|right|300px|Badlands near [[Drumheller, Alberta]] where erosion has exposed the KT boundary.]]Close to 65 million years ago, at the end of the [[Cretaceous]] period, and the beginning of what we call the [[Tertiary]] period, an event occurred which has come to be known as the [[K-T Event]].  This event obliterated most life on Earth, plunging the world into something that would now be likened to global nuclear winter, through which few extant species could survive.  Although scientists dispute the nature of the K-T Event (selecting among any number of catastrophes that could have caused the significant global cooling that resulted), most find, based on strong geological evidence, that the K-T Event was caused by the collision of a massive asteroid with the Earth, the dust and debris from which shrouded the sky for thousands of years, cooling Earth considerably.<ref>Kevin O Pope, "Meteorite impact and the mass extinction of species at the Cretaceous/Tertiary boundary," Proceedings of the National Academy of Science, available at http://www.pnas.org/cgi/content/full/95/19/11028</ref>  The dinosaurs did not survive this cataclysm.<ref>National Geographic, http://news.nationalgeographic.com/news/2002/08/0823_020823_asteroid.html</ref>  Evidence of the dust that caused this long winter is still visible, though, as a line of metal of extraterrestrial origin is often visible in sediment worldwide, occurring as a result of the vaporization and then fall of dust from the meteorite's impact, and its compression within the subsequent geological record.<ref>''Ibid.''</ref>  The crater is also visible.<ref>National Geographic, http://news.nationalgeographic.com/news/2003/03/0307_030307_impactcrater.html</ref>

Close to 65 million years ago, at the end of the [[Cretaceous]] period, and the beginning of what we call the [[Tertiary]] period, an event occurred which has come to be known as the [[K-T Event]].  This event obliterated most life on Earth, plunging the world into something that would now be likened to global nuclear winter, through which few extant species could survive.  Although scientists dispute the nature of the K-T Event (selecting among any number of catastrophes that could have caused the significant global cooling that resulted), most find, based on strong geological evidence, that the K-T Event was caused by the collision of a massive asteroid with the Earth, the dust and debris from which shrouded the sky for thousands of years, cooling Earth considerably.<ref>Kevin O Pope, "Meteorite impact and the mass extinction of species at the Cretaceous/Tertiary boundary," Proceedings of the National Academy of Science, available at http://www.pnas.org/cgi/content/full/95/19/11028</ref>  The dinosaurs did not survive this cataclysm.<ref>National Geographic, http://news.nationalgeographic.com/news/2002/08/0823_020823_asteroid.html</ref>  Evidence of the dust that caused this long winter is still visible, though, as a line of metal of extraterrestrial origin is often visible in sediment worldwide, occurring as a result of the vaporization and then fall of dust from the meteorite's impact, and its compression within the subsequent geological record.<ref>''Ibid.''</ref>  The crater is also visible.<ref>National Geographic, http://news.nationalgeographic.com/news/2003/03/0307_030307_impactcrater.html</ref>

===Cambrian Life Explosion===

===Cambrian Life Explosion===