If you want to understand early man, and guess with some accuracy at why he came to the New World and how he happened to drift southward and eastward until he filled it, you must think of him as a wanderer looking for food. Game lured him on at random, and vegetation lured both beast and man. Man might eat his way through caribou country, and come upon the bison of another area. As he killed and wandered, he might go south or he might go east. But the animal—and the man who ate berries and roots and wild grains as well as meat—would move as the climate moved. The world has known many changes of climate and shifts of rain belts. Some of these have been extreme—in the Great Ice Age, for example—and some have been less marked. But they have moved the forests and the grasslands, and animals and man have moved with the vegetation. During the past 100,000 years, glacialists believe that there were three periods when the inland ice melted sufficiently to allow the southward passage of both animals and man. The first was more than 75,000 years ago in the Sangamon Interglacial period before the time when the last, or Wisconsin, glaciation had covered the plains of Canada (see pages 26 and 27). During the Wisconsin, a corridor probably opened about 50,000 years ago along the eastern foothills of the Rockies, and another, perhaps a little later, down the plateau between the northern Rockies and the Coast Range. The third opportunity for man to penetrate from the north came around 11,000 years ago, when the final retreat of the ice sheets began in those same regions.[17] Perhaps the land-bridge was still usable up to 10,000 years ago, but certainly later migrants had to cross Bering Strait by water or winter ice. Bering Strait and the great glaciers were not the only obstacles to the peopling of the Americas. The Isthmus of Panama must have presented quite a problem to the pioneers who were to fill Amazonia and the Andean Highlands and to reach Cape Horn. Today nobody sets off blithely by foot through the jungle that separates Costa Rica from Colombia. The beach is the best pathway at low tide; but it is an intermittent one. It is better to hope that the shifts of climate which were involved with the glaciers made Panama a drier country than it is today. Of course it was not only early man and his prey that used the routes from Siberia across Bering Strait and through Alaska and Canada. The later migrants—ancestors of the Algonquins, the Athapascans, and others—undoubtedly came in the same way. Problematical Roads to the New World