Clovis spear point
Approximately 15,000 years ago, the Earth started coming out of an ice age. There were glaciers several miles thick in North America which trapped great volumes of water, and the sea level was 130 meters (426 feet) lower than today. As the weather warmed, a land bridge opened between Alaska and Siberia in what is now the Bering strait. This allowed humans to walk from Asia to America and establish new colonies.
At the time, the wildlife in North America was like Africa. There were huge animals like mammoths, giant sloths, camels, and saber tooth tigers. The Clovis people, who had spread throughout what is now the United States, hunted these animals with finely-crafted flint spear points and arrow heads. And then, around 12,900 years ago, all the large animals disappeared from North America, and with them, the Clovis culture.
It was long thought that the Clovis people had annihilated the megafauna, but new studies reveal that the large animals and the Clovis people themselves were destroyed by a comet explosion over south-east Canada, around the great lakes. The explosion was so bright and so intense that it ignited much of the vegetation of North America. The forest fires created a layer of black dirt which became mixed with a thin dusting of microscopic diamonds from the comet.[1,2] The smoke and dust from the impact blocked the light of the sun for many years and the global temperature dropped, creating a rapid return to glacial conditions. Without vegetation, the large herbivores could not survive, and the carnivores were left with nothing to eat. The catastrophe started what is now called the Younger Dryas cool interval, which was a period of cold weather lasting approximately 1300 years.
A band of dark sediment at Murray Spring, Arizona contains evidence for a cosmic impact that started an abrupt period of global cooling and a mass extinction in North America. Similar deposits have been found in five other widely separated locations: Bull Creek, Oklahoma, Gainey, Michigan, Topper, South Carolina, as well as Lake Hind, Manitoba, and Chobot, Alberta, in Canada. The highest concentrations of extraterrestrial impact materials occur in the Great Lakes area.
 D. J. Kennett, et al., Nanodiamonds in the Younger Dryas Boundary Sediment Layer, Science 2 January 2009: Vol. 323. no. 5910, p. 94. DOI: 10.1126/science.1162819 [link]
 University of Cincinnati. “Exploding Asteroid Theory Strengthened By New Evidence Located In Ohio, Indiana.” ScienceDaily 3 July 2008. [link]