第41期:冰川时期

Much of the northern hemisphere is covered by ice sheets up to one and a half miles thick. With trillions of gallons of water locked up as ice, sea levels fall.

A strip of land emerges from the ocean between Siberia and Alaska. It's a bridge between two vast continents, a gateway that takes humans from Asia to a new world, the Americas.

They're the last great continents to be colonized, the last great human migration,

and the first Americans. Now, 14,000 years ago, the changes that triggered the ice age reverse.

As the ice retreats, it reveals a very different northern hemisphere. The huge depressions that the glaciers create fill with water to become North America's Great Lakes.

6,000 years ago, the ice retreats back to the poles, toward the Arctic and Antarctic.