The first Americans crossed into North America from Asia over 30,000 years ago via a land bridge that connected Siberia to Alaska during an ice age. As the climate warmed, sea levels rose and the bridge disappeared under water. These early peoples adapted to diverse environments across North America by developing different ways of life including distinctive housing, tools, clothing and food sources appropriate to each region's climate and available natural resources. Though cultural practices varied regionally, Native Americans also shared some core beliefs like respect for nature and communal ownership of land.