How did the spread of civilization begin in the americas

They moved into Alaska during an Ice Age when there was ice from Eastern Asia and they walked to Alaska

The Asian nomadic hunters food source was starting to move so they migrated with their food source across the land bridge which was only exposed because it was the ice age and when water freezes it lowers so they walked across the bridge to get to the America’s then their food sources started dying out so they had to find new ways to get food so they settled and started civilizations and farmed