The Pleistocene Era or Ice Age began almost 2.5 million years ago continued until 8,000 B.C. During this time, the ice would advance then retreat. When the great glaciers advanced and grew larger, they held so much of the Earth's water that the sea level dropped. When the sea level dropped, a land strip emerged in the area that we now know as the Bering Strait. The first groups that first crossed were probably from Northern China, but over time other groups from different areas crossed. This probably occurred between 30,000 and 40,000 years ago.