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, sea levels rose and the land bridge disappeared. Groups adapted to different environments across North America, developing distinct cultures and ways of life. They utilized natural resources like plants, animals, wood, and clay. Common aspects of Native American cultures included a spiritual connection to nature, not owning land, and only taking what was needed from the environment.