Final answer:
The Bering land bridge was exposed during the glacial periods of the Quaternary Period, especially around eighteen thousand years ago when the last glaciation period peaked, allowing early human migrations to the Americas.
Step-by-step explanation:
The Bering land bridge, known as Beringia, was exposed above sea level during periods of glacial growth when sea levels were significantly lower due to much of the Earth's water being frozen in glaciers. This was particularly true during the last glaciation period, which reached its peak about eighteen thousand years ago. Archaeological finds suggest that the first Homo sapiens may have crossed this bridge to reach the Americas, following herds of megafauna like mammoths and mastodons. Sea levels fell during the Quaternary Period, revealing Beringia and enabling migrations between continents. The exposure of Beringia occurred specifically during the times when Earth was experiencing ice ages within this period.
The difficulty in studying early human migration to the Americas is in part due to the periodic exposure and submersion of areas like Beringia, since much evidence is now underwater. The Pleistocene epoch saw periods when Beringia was accessible, and it is postulated that there may have been as many as four distinct migrations across the land bridge from approximately 10,000-14,000 years before present (BP).