At one time the North American continent belonged to various forms of enormous, fantastic creatures. By the end of the Ice Age, most of these large animals had become extinct. There has yet to be a consensus among scholars as to what exactly can be blamed for this disappearance. Among the theories that have been debated, two are predominant; they are the climatic change theory, and the overkill theory. .
The climatic change theory promotes the idea that the global warming, which brought about the end of the Ice Age, caused the animals to die off. The overkill theory states that humans over hunted the animals, and are thus responsible for the extinctions. It is likely that many of these animals became extinct at the end of the Ice Age due to a combination of climatic change and overkill. .
The Ice Age, which began about one and a half million years ago, is also called the Pleistocene Epoch. The Ice Age consisted of a series of glacial advances and retreats called glaciations and deglaciations. The last glaciation in North America is called the Wisconsin. The Wisconsin glaciation began 122,000 years ago. The advance of the Late Wisconsin began 35,000 years ago. .
The ice sheets, called the Laurentide and the Cordilleran glaciers stretched southward to the middle of the continent, covering most of what is now Canada and the area in the Pacific Northwest from Alaska to Tacoma. On the eastern half of the continent, the ice covered the area to about the middle of where New York is today. Diverse environmental conditions south of the ice sheets remained as they were, but in the northern half of the continent, regions that had once been surfaced with dense forests, fertile land, and abundant game, turned cold, and barren. .
The expanding glaciers in the Ice Age forced sea levels to drop between 330 and 490 feet, opening a passage between Siberia in northeastern Asia and Alaska in North America, called the Bering Land Bridge.