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 then adapted to different environments across North America, developing distinct cultures and ways of life based on the available natural resources in each region. Though cultures varied regionally, most Native American societies shared some common beliefs like respecting nature and only taking what was needed.