IF you Study the Younger Dryas era 12,700 to 12,900 years ago you can see how quickly weather can change here on earth

Whatever happened then (either a comet event or a new type of extreme flooding or whatever it was it created a "Little Ice Age" during a period of warming that lasted about 1000 years killing off most things like Mammoths and Giant Ground Sloths here in the U.S. for example, and likely killed off most humans that were here from about 23,000 years ago to then too. So, it's debatable how many humans were left alive in North America when the land bridge from 400 feet less deep Pacific Ocean allowed a land crossing for many Native Americans into Alaska down through Canada into North America more fully. 

So, the question is: "How many humans were still here from before the Younger dryas "mini Ice age of 1000 years still alive?" However, it is also true that this affected mostly from Mexico North too whereas south America was less affected by death and dying of all species than North America Was from the Younger Dryas "Mini Ice age of 1000 years" which killed off most humans and Megafauna and likely many plants and animals in addition to this.