An assumption. True or false - only time will tell. If you are young enough, you might live to learn that they knew far more than they are credited with currently - or not. Keep in mind that the likely habitable areas during that time are 400-700 feet below sea level today.
Underwater exploration by paleontologists is virtually nonexistent, and only just beginning by archaeologists. No telling what will or will not be found 50 years from now. In the past 50 years the entire period preceding 2000 years ago has changed radically.
The *Out of Africa* theory could just as easily be *Out of the Americas*.