The last ice age started over 70,000 years ago, and ended 14,000 years ago. During this period, the vast majority of North America was covered in several kilometers of ice. Since these glaciers contained a great deal of water, ocean levels were lower and some of the land that is underwater today, such as the space between Siberia and Alaska, could be used by humans and animals.
This is how, to follow herds of caribou, bison or muskox, the first inhabitants crossed from Asia to America. This happened at the very end of the ice age, as glaciers began to melt, forming permanent lakes and allowing plants and trees to grow.
The First Peoples therefore followed animal migrations, eventually settling throughout the entire territory of America.