About 120,000 years ago, the Earth fell into an ice age. The northern polar ice cap grew southward, water solidified into ice, and ocean levels fell. With the lowering of the oceans, hidden land was exposed, including a land bridge connecting Siberia (located in modern-day eastern Russia) and Alaska. Between 15,000 and 50,000 years ago, various small, nomadic hunting groups from Asia crossed the land bridge, becoming the first human inhabitants of the Americas.

Over the next millenia, these earliest Americans dispersed across much of the Western Hemisphere. As the Ice Age came to an end around 10,000 years ago and Earth’s atmosphere warmed, the land that these groups inhabited changed dramatically. Sea levels rose and melting glaciers filled the Great Lakes and Mississippi River Basin with water. Glaciers receded northward, and frozen plains gave way to deciduous eastern forests, grassy central plains, and desert throughout the West. In time, the land bridge disappeared back under the body of water now known as the Bering Strait.

The descendants of the earliest Americans changed with the landscape. As Ice Age animals such as woolly mammoths disappeared, hunters began to prey on smaller game. The groups also fished and gathered local provisions, like seeds and nuts, from the land. About 5,000 years ago, some groups began to domesticate plants. As these groups learned how to farm and more efficiently use natural resources, they required less land, and many in the East and Midwest gave up their nomadic lifestyle and established small, stable communities by around 300 B.C.