The first Americans crossed into North America from Asia over a land bridge between Siberia and Alaska around 30,000 years ago during an ice age. As the climate warmed, glaciers melted and sea levels rose, submerging the land bridge. Native Americans then adapted to different environments across North America, developing distinct cultural practices and ways of life based on local natural resources and conditions. They shared some common beliefs, including that nature has a spirit, land cannot be owned, and only what is needed should be used.