What environmental change allowed people to travel by land between Asia and North America?

The land travel between Asia and North America has been made possible by the Bering land bridge which was used over 20,000 years ago. Though the environment is a very cold one, a lot of migrants die enroute. Thus, the journey can be made more successful if human system can adapt to artic environments.  

Explanation:

landlessness of ordinary people

^^something like that^^^

answerpeople organize large projects such as levee-building;