Final answer:
The land bridge crossed by the ancestors of Native Americans is known as Beringia. It connected Asia and North America during the last ice age, facilitating human migration to the Americas.
Step-by-step explanation:
The land bridge that ancestors of Native Americans are believed to have crossed is known as Beringia. During the last ice age, approximately 15,000 years ago, this land bridge connected Asia and North America, allowing humans to follow migratory herds and eventually populate the Americas. As the ice melted and sea levels rose, Beringia became submerged, marking the formation of the Bering Strait. While some alternate migration theories exist, such as the Solutrean hypothesis, the Bering Land Bridge migration model remains the most widely accepted based on current genetic and archaeological evidence.