About 2,011,500 years ago, great sheets of ice called glaciers covered much of the northern hemisphere. The glaciers were up to ten thousand feet deep. The sea level became lower because much of the earth's water made up the glacier. Many lands that were underwater-and are underwater again today-became dry. One such land lay between Siberia and Alaska where the Bering Strait now separates Asia and North America. So scientists now believe they came through that piece of land. Nobody really knows when these people came to the Americas but scientists believe that most of them came 15000 years ago. The glaciers have been melted as an effect of global warming and all the land including the Bering Strait was covered by water again. The piece of land on which the Native Americans came to the Americas by from Asia was covered by water.