阅读理解Colonizing the Americas via the Northwest Coast It has long been accepted that the Americas were colonized by a migration of peoples from Asia, slowly traveling across a land bridge called Beringia (now the Bering Strait between northeastern Asia and Alaska) during the last Ice Age

阅读理解Cave Art in Europe The earliest discovered traces of art are beads and carvings, and then paintings, from sites dating back to the Upper Paleolithic period

阅读理解Desert Formation The deserts, which already occupy approximately a fourth of the Earths land surface, have in recent decades been increasing at an alarming pace

阅读理解Some animal behaviorists argue that certain animals can remember past events, 　　anticipate future ones, make plans and choices, and coordinate activities within 　　a group. These scientists, however, are cautious about the extent to which animals 　　can be credited with conscious processing. (5) Explanations of animal behavior that leave out any sort of consciousness at 　　all and ascribe actions entirely to instinct leave many questions unanswered. 　　One example of such unexplained behavior: Honeybees communicate the sources of 　　nectar to one another by doing a dance in a figure-eight pattern. The orientation 　　of the dance conveys the position of the food relative to the sun''s position in the sky, (10)and the speed of the dance tells how far the food source is from the hive. Most 　　researchers assume that the ability to perform and encode the dance is innate and shows 　　no special intelligence. But in one study, when experimenters kept changing the site of the 　　food source, each time moving the food 25 percent farther from the previous site, foraging 　　honeybees began to anticipate where the food source would (15)appear next. When the researchers arrived at the new location, they would find the 　　bees circling the spot, waiting for their food. No one has yet explained how bees, 　　whose brains weigh four ten-thousandths of an ounce, could have inferred the location 　　of the new site. Other behaviors that may indicate some cognition include tool use. Many (20)animals, like the otter who uses a stone to crack mussel shells, are capable of using objects in the 　　natural environment as rudimentary tools. One researcher has found that mother chimpanzees 　　occasionally show their young how to use tools to open hard nuts. In one study, chimpanzees 　　compared two pairs of food wells containing chocolate chips. One pair might contain, say, five 　　chips and three chips, the other (25)our chips and three chips. Allowed to choose which pair they wanted, the 　　chimpanzees almost always chose the one with the higher total, showing some sort of summing 　　ability. Other chimpanzees have learned to use numerals to label quantities of items and do simple sums.

阅读理解The Expression of Emotions Joy and sadness are experienced by people in all cultures around the world, but how can we tell when other people are happy or despondent? It turns out that the expression of many emotions may be universal

阅读理解Running Water on Mars Photographic evidence suggests that liquid water once existed in great quantity on the surface of Mars

阅读理解Loie Fuller The United States dancer Loie Fuller (18621928) found theatrical dance in the late nineteenth century artistically unfulfilling

阅读理解Deer Populations of the Puget Sound Two species of deer have been prevalent in the Puget Sound area of Washington State in the Pacific Northwest of the United States

阅读理解Timberline Vegetation on Mountains The transition from forest to treeless tundra on a mountain slope is often a dramatic one

阅读理解Scientists have discovered that for the last 160,000 years, at least, there 　　has been a consistent relationship between the amount of carbon dioxide in the air 　　and the average temperature of the planet. The importance of carbon dioxide in 　　regulating the Earth''s temperature was confirmed by scientists working in eastern (5) Antarctica. Drilling down into a glacier, they extracted a mile-long cylinder of ice 　　from the hole. The glacier had formed as layer upon layer of snow accumulated year 　　after year. Thus drilling into the ice was tantamount to drilling back through time. 　　The deepest sections of the core are composed of water that fell as snow 160,000 years 　　ago. Scientists in Grenoble, France, fractured portions of the core and temperature and of atmospheric (10)measured the composition of ancient air released from bubbles in the ice. Instruments 　　were used to measure the ratio of certain isotopes in the frozen water to get an idea 　　of the prevailing atmospheric temperature at the time when that particular bit of 　　water became locked in the glacier. The result is a remarkable unbroken record of (15)levels of carbon dioxide. Almost every time the chill of an ice age descended on the planet, carbon 　　dioxide levels dropped. When the global temperature dropped 9F (5℃), carbon dioxide levels 　　dropped to 190 parts per million or so. Generally, as each ice age ended and the Earth basked in a 　　warm interglacial period, carbon dioxide levels were around 280 parts per million. Through the 　　160,000 years of that ice (20)record, the level of carbon dioxide in the atmosphere fluctuated between 190 and 280 parts per 　　million, but never rose much higher-until the industrial Revolution beginning in the eighteenth 　　century and continuing today. There is indirect evidence that the link between carbon dioxide levels 　　and global temperature change goes back much further than the glacial record. Carbon (25)dioxide levels may have been much greater than the current concentration during the Carboniferous 　　period. 360 to 285 million years ago. The period was named for aprofusion of plant life whose 　　buried remains produced a large fraction of the coal deposits that am being brought to the surface 　　and burned today.

阅读理解he word "shielded" in line 22 is closest in meaning to

阅读理解Variations in the Climate One of the most difficult aspects of deciding whether current climatic events reveal evidence of the impact of human activities is that it is hard to get a measure of what constitutes the natural variability of the climate

阅读理解The word "constituent” in line 23 is closest in meaning to

阅读理解Geology and Landscape Most people consider the landscape to be unchanging, but Earth is a dynamic body, and its surface is continually altering-slowly on the human time scale, but relatively rapidly when compared to the great age of Earth (about 4,500 billion years)

阅读理解Why does the author mention "block, roller, or screen" in line 25 ?

阅读理解The Rise of Teotihuacn The city of Teotihuacn, which lay about 50 kilometers northeast of modern-day Mexico City, began its growth by 200-100 B

阅读理解Many prehistoric people subsisted as hunters and gatherers. Undoubtedly, game 　　　 animals, including some very large species, provided major components of human diets. 　　　 An important controversy centering on the question of human effects on prehistoric wildlife 　　　 concerns the sudden disappearance of so many species of large animals at or near the end (5) of the Pleistocene epoch. Most paleontologists suspect that abrupt changes in climate led 　　　 to the mass extinctions. Others, however, have concluded that prehistoric people drove 　　　 many of those species to extinction through over-hunting. In their "Pleistocene overkill 　　　 hypothesis," they cite what seems to be a remarkable coincidence between the arrival of 　　　 prehistoric peoples in North and South America and the time during which mammoths, (10) giant ground sloths, the giant bison, and numerous other large mammals became extinct. 　　　 Perhaps the human species was driving others to extinction long before the dawn of history. 　　　 Hunter-gatherers may have contributed to Pleistocene extinctions in more indirect 　　　 ways. Besides over-hunting, at least three other kinds of effects have been suggested: 　　　 direct competition, imbalances between competing species of game animals, and early (15) agricultural practices. Direct competition may have brought about the demise of large 　　　 carnivores such as the saber-toothed cats. These animals simply may have been unable 　　　 to compete with the increasingly sophisticated hunting skills of Pleistocene people. 　　　 Human hunters could have caused imbalances among game animals, leading to the 　　　 extinctions of species less able to compete. When other predators such as the gray wolf (20) prey upon large mammals, they generally take high proportions of each year s crop of 　　　 young. Some human hunters, in contrast, tend to take the various age-groups of large animals 　　　 in proportion to their actual occurrence. If such hunters first competed with the larger 　　　 predators and then replaced them. they may have allowed more young to survive each year, 　　　 gradually increasing the populations of favored species As these populations expanded, (25) they in turn may have competed with other game species for the same environmental niche, 　　　 forcing the less hunted species into extinction. This theory, suggests that human hunters 　　　 played an indirect role in Pleistocene extinctions by hunting one species more than another.

阅读理解Seventeenth-Century European Economic Growth In the late sixteenth century and into the seventeenth, Europe continued the growth that had lifted it out of the relatively less prosperous medieval period (from the mid 400s to the late 1400s)

阅读理解Archaeological discoveries have led some scholars to believe that the first Mesopotamian 　　　 inventors of writing may have been a people the later Babylonians called Subarians. According to 　　　 tradition, they came from the north and moved into Uruk in the south. By about 3100B.C, They 　　　 Were apparently subjugated in southern Mesopotamia by the Sumerians, whose name became (5) 　synonymous with the region immediately north of the Persian Gulf, in the fertile lower valleys of 　　　 the Tigris and Euphrates. Here the Sumerians were already well established by the year 3000B.C. 　　　 They had invented bronze, an alloy that could be cast in molds, out of which they made tools and 　　　 weapons. They lived in cities, and they had begun to acquire and use capital. Perhaps most 　　　 important, the Sumerians adapted writing (probably from the Subarians) into a flexible tool of (10)　 communication. 　　　 Archacologists have known about the Sumerians for over 150 years. Archacologists working at 　　　 Nineveh in northern Mesopotamia in the mid-nineteenth century found many inscribed clay tablets. 　　　 Some they could decipher because the language was a Semitic one (Akkadian), on which scholars 　　　 had already been working for a generation. But other tablets were inscribed in another language (15)　 that was not Semitic and previously unknown. Because these inscriptions mad reference to the 　　　 king of Summer and Akkad, a scholar suggested that the mew language be called Sumerian. 　　　 But it was not until the 1890''s that archaeologists excavating in city-states well to the south of 　　　 Nieveh found many thousands of tablets inscribed in Sumerian only. Because the Akkadians 　　　 thought of Sumerian as a classical language (as ancient Greek and Latin are considered today), (20)　 they taught it to educated persons and they inscribed vocabulary, translation exercised, and other 　　　 study aids on tablets. Working from known Akkadian to previously unknown Sumerian, scholars 　　　 since the 1890''s have learned how to read the Sumerian language moderately well. Vast quantities 　　　 of tablets in Sumerian have been unearthed during the intervening years from numerous sites.

阅读理解Feeding Habits of East African Herbivores Buffalo, zebras, wildebeests, topi, and Thomsons gazelles live in huge groups that together make up some 90 percent of the total weight of mammals living on the Serengeti Plain of East Africa