| Humans are believed to have crossed into the Americas before the end of the last ice age, when a natural land bridge called Beringia linked present-day Siberia and Alaska. During the coldest part of the ice age, about 23,000 to 19,000 years ago,
vast glaciers blanketed much of the northern hemisphere, making travel south toward what is now the continental United States virtually impossible. By about 14,000 years ago, as the climate warmed, a passable route opened along the Pacific Coast. Then, about 2,500 years later, an ice-free corridor opened in the contintental interior as the great ice sheets retreated. Some scientists believe both routes were used by early migrating peoples.
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