The first Americans crossed into North America from Asia over a land bridge between Siberia and Alaska around 30,000 years ago during the last ice age. As the climate warmed, sea levels rose and the land bridge disappeared, leaving groups of people isolated in different environments across North and South America. These groups developed diverse cultures and ways of life based on the unique natural resources of their regions. Though varied, most Native American cultures shared some common beliefs like respect for nature and limited consumption of resources.