There are 2 answers. There is some proof that the sea level was actually much lower only 20-30k years ago leaving many land bridges that no longer exist. For America specifically the ice age allowed people to migrate towards the American continents. Specifically from Russia to the Alaskan peninsula is relatively a short trip. But also allowed the people in Asia to travel to the many pacific islands on shorter boat trips compared to now.
Supposedly there is also proof of much earlier humans traveling to the Americas in an earlier ice age.
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