It was once widely accepted that the first people in the Americas arrived by walking across a land bridge from Siberia. They then traveled south between great sheets of ice that 1 ____ across North America at that time. Today, this theory is being challenged. An alternative idea suggests that instead of a single first migration, groups came at separate 2 ____ . Another theory suggests that they may have 3 ____ their way along the shoreline using kayaks.The debate over this migration path is one of many disputes in the field of archeology. Evidence from the distant past is hard to find, so theories are often based on very small 4 ____ . As new evidence is uncovered that 5 ____ existing ideas, experts often need to adjust their theories.

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