新gre考试阅读机经之美洲人起源
美洲人起源
Recent discoveries in New World archaeology along with new scientific methods for analyzing data have led to new ideas regarding the origin of the first peoples of the Americas and their time of arrival.中?华 考?试 网
The traditional theory held that the first Americans crossed the land bridge from Siberia to Alaska around 11,500 years ago and followed an "ice-free corridor" between two large Canadian ice sheets (the Laurentide and Cordilleran) to reach unglaciated lands to the south.
These first inhabitants, whose archaeological sites are scattered across North and South America, were called the Clovis people, named after the town in New Mexico where their fluted spear points used for hunting mammoth were first found in 1932.
There is now convincing evidence of human habitation sites that date earlier than the Clovis culture including sites located in South America. Monte Verde, a well-studied site located along a river near southern central Chile, dates 12,500 years ago. This site contains the buried remnants of dwellings, stone tools including large bifacial projectile points, and preserved medicinal and edible plants. How did people manage to settle this far south at such an early date? A coastal migration route is now gaining more acceptance, rather than the older view of small bands moving on foot across the middle of the land bridge between Siberia and Alaska and into the continents. Emerging evidence suggests that people with boats moved along the Pacific coast into Alaska and northwestern Canada and eventually south to Peru and Chile by 12,500 years ago—and perhaps much earlier. Archaeological evidence in Australia, Melanesia, and Japan indicate boats were in use as far back as 25,000 to 40,000 years ago. Sea routes would have provided abundant food resources and easier and faster movement than land 8 routes. Many coastal areas were unglaciated at this time, providing opportunities for landfall along the way. Several early sites along the coast of Canada, California, Peru, Ecuador, andChile date between 10,000 and 12,000 years ago. Many potential coastal sites are now submerged, making investigation difficult.
6. Honey bee 的 fungee? Ingestion of food containing spores of the pathogen Ascosphaera apis causes a fatal fungal disease known as chalk brood in honeybee larvae. However, larvae must be chilled to about 30°C(normal brood-comb temperature is 33-36°C)for the disease to develop. Accordingly, chalk brood is most common in spring and in small colonies. A recent study revealed that honeybees responsible for hive-temperature maintenance purposely raised the hives’ temperature when colonies were inoculated with A.apis this “fever,” or up-regulation of temperature, occurred before any larvae died, suggesting that the response is preventative and that either honeybee workers detect the infection before symptoms are visible or larvae communicate the ingestion of the pathogen. Temperature returned to normal by the end of the study, suggesting that increased temperature is not optimal when broods are not infected, as well as that the fever does not result merely from normal colony growth (i.e., an increase in the number of workers available for temperature maintenance).
6.1. The primary purpose of the passage is to
(A) discuss the findings and implications of a particular study
(B) illustrate a process that formerly had been misunderstood
(C) outline the methods used to investigate a problem
(D) provide evidence to support a controversial theory
(E) contrast alternative interpretations of certain date
6.2. According to the passage, researchers concluded that fever in honeybee coloniesis preventative because their study showed that such fever
(A) does not occur when hive temperatures are within normal range
(B) protests adult bees from contracting chalk brood infection
(C) occurs prior to the death of any larvae
(D) is more likely to occur in spring than in summer
(E) does not have an effect on uninfected broods
6.3. The passage implies that if hive temperature had not returned to normal by the end of the study in question, a probable conclusion of the researchers would have been that
(A) up-regulation of temperature is a preventative measure against chalkbrood
(B) honeybees are incapable of purposely raising hive temperatures
(C) A. apis cannot be completely eradicated throughup-regulation of temperature along
(D) honeybee larvae have a mechanism to alert adult honeybees to the presenceof A. apis
(E) honeybee larvae may benefit from increased hive temperature even when there is no A.apis present
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6.4. According to the passage, which of the following is true of chalk brood infection among
honeybee larvae?
(A) Larvae in small colonies are more likely to pass the infection to adult honeybees than are
larvae in large ones.
(B) Infection with chalk brood induces larvae to raise their hive’s temperature.
(C) The infection is more likely to affect larvae in winter than in spring.
(D) Larvae fail to develop symptoms of the disease when their brood –comb temperature
remains within the normal range.
(E) Infected larvae exhibit visible symptoms of disease for a significant time before death.
答案：ACED
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