The first Americans crossed into North America from Siberia over a land bridge that connected Asia to North America around 30,000 years ago during an ice age. As the climate warmed, sea levels rose and the land bridge disappeared. These early peoples adapted to different environments across North America, developing distinct cultural practices and traditions depending on available natural resources and climate conditions in each region. Overall, Native American cultures shared some common beliefs, like a spiritual connection to nature, communal land ownership, and an emphasis on sustainability.