Don't know about the DNA but White Sands Footprints were dated over 20,000 years old, someone arrived before land bridge was available.

General rule in wildlife biology 50/500. Need 50 individuals to prevent extinction, 500 to have potential to evolve/survive threats.

Here are a couple papers that say those numbers are too low but I don't have access to free copies. Second one a little beyond my understanding.

https://www.sciencedirect.com/science/article/abs/pii/S0006320713004576?via=ihub

https://www.sciencedirect.com/science/article/abs/pii/S0006320706002126?via=ihub