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Speaking of 13,000 years...a large "Clovis" population was wiped out by the cataclysm of 10,900 B.C. (13KYA) which formed the Great Lakes.
There is evidence of a firestorm engulfing the entire atmosphere at that time in North America, pushing survivors south of the border from whence they eventually migrated back.
Alaska land bridge is a fantasy of anthropologists not permitted (by their profession) to consider cataclysms in Earth's history.
Before the 10,900 B.C. event, there were many large animals in America, including sabretooth tigers, mammoths etc. All were exterminated along with the humans and all large fauna as well.
In the Eastern Hemisphere, extinction of large animals took about 500 years longer.
The 10,900 B.C. event caused the climatological period known as the Younger Dryas, wherein average temperature of the Earth dropped by 18 degrees (Fahrenheit, I think).
In the 1970s we started to hear about "Nuclear winter," which was described as a significant lowering of whole Earth's temperature due to particles blocking the Sun after a nuclear exchange. HOWEVER, nuclear winter was only supposed to last about a year until the atmosphere relieved itself of the obstruction.
Younger Dryas lasted 1,500 years, until about 9,500 B.C. This indicates that if it was caused by particles launched in the Great Lakes cataclysm, then that cataclysm was much larger than hydrogen bombs. It may have blown the sun-obscuring particles into stratospheric orbit where they would remain until something came along to clean them up. ("Let there be light?")
There is also evidence that the Earth's axis of rotation may have been changed in the 10,900 B.C. cataclysm, as some believe the Hudson Bay basin of Canada was the north pole back then, sporting an ice cap like Antarctica today.