The First Americans
There was a time long ago when a land bridge connected Asia and North America. The first settlers in North America crossed this land bridge from what is now Siberia to Alaska. The people who made this initial(最初的) crossing were probably following the animals they used for food. Eventually, the oceans rose and the land bridge between the continents disappeared. The settlers, who I shall call the “First Americans”， could not return to their homeland. _小题1:__
For the next few thousand years, the First Americans migrated（迁徙） south and east throughout North and South America. They made the journey all the way from Alaska to the southern tip of South America, a distance of more than 10, 000 miles. __小题2:__In the far north, above the Arctic Circle, the First Americans became hunters of the ocean.  __小题3:__ In the brief summer，they gathered berries and other plants. In the winter, they stayed in lodges much of the time preparing for the next summer. It was a hard life，but they learned to survive in these harsh conditions.
Farther south, in what is now Arizona, Utah, Colorado, and New Mexico, the First Americans became cliff dwellers(悬崖居民). They began by living in caves that already existed in the cliffs.__小题4:__ The dwellings(住处) they constructed in many ways resembled today's apartment buildings.__小题5:__ They built a system of dams and canals so they could store and use water almost any time of the year. They were so successful that they could raise fruits and vegetables that were not found naturally in the desert. In Mexico and South America, the First Americans founded civilizations that were among the greatest in the world. They built huge pyramids, lived in fabulous cities, and made great advances in science and mathematics long before the Europeans arrived.

A．They also discovered foods like corn and potatoes.

B．Along the way，they established（建立） many different civilizations.

C．The First Americans of the Southwest learned to irrigate（灌溉） the desert.

D．They would have to survive in the New World they had discovered.

E. They built boats of animal skins and hunted or fished for their food.
F. People living near each other tended to share similar environments and customs.
G. They discovered how to add to the caves by building structures of wood and stone.