But no humans had yet set foot in the Americas. … They got their chance thanks to major changes in climate. First, about 20,000 years ago, an intensification of the Ice Age locked up a great deal of water in ice-sheets and glaciers, leading to a huge fall in sea level. The sea between Russia and Alaska (the Bering Strait) became a wide and easily passable land bridge.

Neil MacGregor, A History of the World in 1…, loc. 122