Together, they formed a virtually impassable barrier across the northern part of the continent.

At the same time, the land connection between Siberia and Alaska — Beringia — was wide open.

Sea levels were much lower than today because so much of the world’s water was locked up in ice.

The land bridge was not a narrow corridor but a broad, flat, vegetated landmass extending hundreds of miles from north to south.

It was inhabited by large mammals — mammoths, horses, bison — and, according to the standstill hypothesis, by people.

The standstill hypothesis proposes that the founding population of the Americas — the ancestral group that would eventually give rise to all Native American peoples — was present in Beringia during the Last Glacial Maximum but could not move south because of the ice sheets.