The first Americans crossed into North America from Asia over a land bridge between Siberia and Alaska during the last ice age approximately 30,000 years ago. As the climate warmed, sea levels rose and the land bridge disappeared. Native Americans adapted to different environments across North America by developing diverse cultures and ways of life to make use of local natural resources for food, clothing, shelter and tools. Though cultures varied regionally, most Native American groups shared a spiritual connection to nature and belief that land could not be owned.