How did the ice age isolate human population of the Americas from that of Asia?

Answer 1

It was the end of the last glaciation which isolated humans in the Americas from the rest of the world.

During the latter part of the glaciation, people were able to enter the Americas from Asia because of the lowered sea levels. As the ice melted, the sea levels rose turning an area of land into a number of islands separated by sea. This effectively prevented more people from moving into the Americas until Inuit were later able to travel across from Siberia after the great thicknesses of ice had disappeared.

Answer 2

During the last Ice Age, large glaciers covered much of North America, creating a land bridge called Beringia that connected Asia and the Americas. As the glaciers melted, sea levels rose and the land bridge disappeared, isolating the populations on each continent. This isolation led to the development of distinct human populations in the Americas and Asia.