The Paleoindians were the first groups of populations who inhabited the American continent as a result of the events of the glacial period between 45,000 and 12,000 BCE. This period created a glacial "bridge" between what is now Russia and Alaska. This made it possible for people from that age to cross all the way to the American Continent.
The arrival of the Archaic Period saw the continent experience more mild weather. This made it possible for people to settle within a certain geographic latitude, as it was no longer necessary to migrate in search for warmer weather due to the harsh weather conditions of the glacial period. It also meant that vast extensions of terrain would now be available for crop cultivation, which would be used as a source of food for these people.
By their culture practices and traditions which passed on to their offspring
One is environmental: that a combination of overgrazing and drought caused the soil on the Zimbabwe Plateau to become exhausted. ... The other explanation is that the people of Great Zimbabwe had to move in order to maximise their exploitation of the gold trade network. By 1500 the site of Great Zimbabwe was abandoned
