How did the early humans use the land bridge?

During a glaciation, water evaporated from the oceans falls on the land in higher latitudes as snow. If the snow doesn't melt during successive cold summers it builds up and turns to ice. Repeated evaporation and snowfall causes the oceans to lose water over many years, this causes the level of the oceans to drop, revealing more land above sea level. What was previously a shallow sea becomes dry land, hence a "land bridge".