Final answer:
The land bridge called Beringia connected Siberia with Alaska and facilitated the migration of humans to the Americas around 20,000 years before present.
Step-by-step explanation:
The land bridge that connected Siberia with Alaska and was used by some humans to walk to Alaska is known as Beringia. Around 20,000 years before the present, during the last glaciation period when sea levels were significantly lower, humans migrated from Siberia, Eurasia, and coastal Asia, following the megafauna like mammoth and mastodon. The presence of Beringia enabled these first peoples to enter the Americas, marking one of the earliest migrations into the Western Hemisphere.
Scientific theories of origin suggest there may have been as many as four distinct migrations over this land bridge, from about 10,000 to 14,000 BP. As the glacial ice retreated, these pioneering humans spread out to populate different regions of the Americas, with evidence of habitation reaching as far as the southern tip of South America.