Evidence suggests big-game hunter gatherers crossed the Bering Strait from Asia (Eurasia) into North America over a land bridge (Beringia), that existed between 47,000–14,000 years ago. Around 18,500-15,500 years ago, these hunter-gatherers are believed to have followed herds of now-extinct Pleistocene megafauna along ice-free corridors that stretched between the Laurentide and Cordilleran ice sheets. Another route proposed is that, either on foot or using primitive boats, they migrated down the Pacific coast to South America.