Long ago the land was mostly covered in glaciers. So, with much of the world's water caught up in ice blankets and frozen into glaciers, the oceans weren't quite as deep as they are today, meaning that more land was exposed along the coastlines. In areas where land was exposed, sunshine helped short grasses and shrubs grow which made a perfect snack for hungry mammoths. Before they knew it, humans were following and hunting these mammoths while leaving Asia and walking the "ocean floor" on their way to a new land- America!