Final answer:
The first Americans migrated from Asia across the land bridge called Beringia to the Americas. This migration followed the herds of large Pleistocene mammals and continued even after the area was submerged, forming the Bering Strait.
Step-by-step explanation:
The first Americans are believed to have migrated from Asia to North America. This migration occurred via a land bridge known as Beringia, which connected what are now Siberia and Alaska. The first inhabitants of the Americas pursued mammoth and mastodon herds across this land bridge according to the Bering Land Bridge theory. They reached the Americas and spread throughout the continent, diversifying into various cultures and societies as they adapted to different environments.
Later evidence also supports the idea that as the glacier ice melted and sea levels rose, the water engulfed Beringia, forming the Bering Strait we know today. This did not stop the human migrations, as people continued to arrive, possibly even by boat across the narrowed strait, spreading out over what would become the continental United States and the rest of the Americas.