What factors allowed for the development of empires in central Mexico and the Andes?Archaeologists believe that migrants from Asia crossed a 100-mile-wide land bridge connecting Siberia and Alaska during the last Ice Age sometime between 13,000 and 3000 b.c. and thus became the first Americans. The first wave of this migratory stream from Asia lasted from about fifteen thousand to nine thousand years ago. Then the glaciers melted, and the rising ocean submerged the land bridge beneath the Bering Strait (Map 1.1). Around eight thousand years ago, a second movement of peoples, traveling by water across the same narrow strait, brought the ancestors of the Navajos and the Apaches to North America. The forebears of the Aleut and Inuit peoples, the “Eskimos,” came in a third wave around five thousand years ago. Then, for three hundred generations, the peoples of the Western Hemisphere were largely cut off from the rest of the world. Migrants moved across the continents as they hunted and gathered available resources. Most flowed southward, and the densest populations developed in central Mexico — home to some 20 million people at the time of first contact with Europeans — and the Andes Mountains, with a population of perhaps 12 million. In North America, a secondary trickle pushed to the east, across the Rockies and into the Mississippi Valley and the eastern woodlands.Around 6000 B.C., Native peoples in present-day Mexico and Peru began raising domesticated crops. Mesoamericans cultivated maize (corn) into a nutritious plant with a higher yield per acre than wheat, barley, or rye, the staple cereals of Europe. In Peru they also bred the potato, a root crop of unsurpassed nutritional value. The resulting agricultural surpluses encouraged population growth and laid the foundation for wealthy, urban societies in Mexico and Peru, and later in the Mississippi Valley and the southeastern woodlands of North America (Map 1.2).