The first Americans crossed into North America from Siberia over a land bridge that connected Asia to North America during the last ice age approximately 30,000 years ago. As the climate warmed, sea levels rose and the land bridge disappeared under water. These early peoples adapted to their new environments by developing different tools, clothing, shelters, and ways of life suited to the available natural resources in their regions. Though cultural practices varied between regions, Native Americans universally believed that nature had a spirit, no one could own land, people should only use what they needed, and trade was important between societies.