During the Pleistocene "Ice Age," which began 2 million years ago, four major glaciations occurred, separated by warm interglacial periods; whether the Pleistocene ended 10,000 years ago or if the current Holocene Period is just another interglacial phase remains a subject of debate among climatologists.
The most recent Pleistocene glaciation, the Wisconsin, began 70,000 years ago; at its peak expansion, about 23,000 years ago, one third of Earth's land mass was covered with ice. Since there is a finite amount of water on our planet, this extensive ice formation was accompanied by a dramatic fall in sea level; at the peak of the Wisconsin glaciation, sea level was 400 feet lower than it is today and the coastline of North America was 100 miles east of New York City. Elsewhere on the planet, land bridges opened between continents and islands, allowing humans and other animals to migrate across the globe.
DNA evidence suggests that the low sea level of the Pleistocene permitted man to walk from East Africa to Yemen, perhaps as early as 70,000 years ago, triggering the colonization of southern Asia. The major islands of the Indonesian Archipelago became a broad peninsula, allowing man to approach Australia by 65,000 years ago; he managed to cross the remaining ocean barrier (perhaps by accident) within 5000 years and then reached Tasmania via another land bridge. About 50,000 years ago, humans colonized Japan after migrating across a land bridge that extended eastward from the Asian coast. Finally, the most famous Pleistocene land bridge, Beringia, connected Siberia and Alaska, allowing humans, bison and mammoths to reach North America and, in the reverse direction, horses to enter Asia.