In prehistoric times, a land bridge joined Alaska to Asia where the Bering Strait now lies. Across this bridge, most archeologists believe, came Asiatic peoples—ancestors of the Indians—who migrated to the North and South American continents 12,000 to 14,000 years ago. Probably 8,000 to 5,000 years ago the ancestors of the Eskimos and the Aleuts also crossed the land bridge to Alaska. The European discovery of Alaska did not come until the 18th century.
At least 10,000 years ago, prehistoric people came to what is now Illinois. Archeological excavations at the Koster Site in the lower Illinois River valley have revealed evidence of almost continuous habitation there by advanced Indian cultures from about 8000 B.C.
Remains of Ice Age settlements dating back more than 10,000 years have been found in Pennsylvania.