How did an ice age make it possible for Asian hunters to migrate to the Americas?

As the glaciers expanded, the levels of the seas dropped - typically 30 feet or more. One thing that this did was to create a narrow land bridge from Eastern Russia to Alaska that is now underwater, with the exception of the Aleutian Islands. Virtually all anthropologists today accept this as the origin of the nomadic peoples that evolved into what we now call the Native American Tribes in the USA.