in the Settlement of the Americas, there appears to be a gap in the immigration chronology of twenty thousand years.
The first is the short chronology theory, with the first movement (across the Beringia land bridge) beyond Alaska into the New World occurring no earlier than 15,000–17,000 years ago, followed by successive waves of immigrants. The second belief is the long chronology theory, which proposes that the first group of people entered the hemisphere at a much earlier date, possibly 21,000–40,000 years ago, with a much later mass secondary wave of immigrants. [source WIKIPEDIA]
Now, just because there's potential for such an early wave of immigration, doesn't mean they 'did'. It's later explained how ice sheets could have blocked further southerly investigation of the Americas.
Migrants from northeastern Asia could have walked to Alaska with relative ease when Beringia was above sea level. But traveling south from Alaska to the rest of North America may have posed significant challenges. The two main possible southward routes proposed for human migration are: down the Pacific coast; or by way of an interior passage (Mackenzie Corridor) along the eastern flank of the Rocky Mountains. [source WIKIPEDIA]