The common ancestors of the Yupik and the Aleut (as well as various Paleo-Siberian groups) are believed by archaeologists to have their origin in eastern Siberia. Migrating east, they reached the Bering Sea area about 10,000 years ago. Research on blood types and linguistics suggests that the ancestors of American Indians reached North America in waves of migration before the ancestors of the Eskimo and Aleut; there were three major waves of migration from Siberia to the Americas by way of the Bering land bridge. This causeway became exposed between 20,000 and 8,000 years ago during periods of glaciation.