New mutations accumulated during the standstill.

Genetic variants that were present in the wider Asian population but were absent from the Beringian founding population were lost by genetic drift.

Allele frequencies shifted in directions specific to the isolated population.

The result was a founding population that was genetically distinct from the Asian populations it had originally been part of — distinct enough to be recognizable as specifically American in population genetic terms, even when we are comparing genomes from people who lived eleven thousand years ago.

The Ancient Beringian lineage — the branch that USR1 belonged to — apparently remained in or near the Beringian homeland for an extended period, while the ancestors of most other Native Americans moved south through the ice-free corridors and coastal routes that became available as the glaciers retreated.