tpo36阅读文本无水印

tpo36阅读文本无水印内容摘要：

they seem much too large and (potentially) well developed to be brand new inventions. The best we can do is put the origin of eyes somewhere between the beginning of the Cambrian explo sion, about 600 million years ago, and the death of the Burgess animals, some 530 million years ago. 12. Paragraph 6 suggests that the first eyes probably  Came into existence long before 600 million years ago  Came into existence at a late point in the Cambrian period  Existed before the animals of the Burgess Shale existed  Were larger than those of animals found in the Burgess Shale Paragraph 1: Putting a date on the first appearance of eyes depends on what one means by eye. If the term refers to a multicellular an, even if it has just a few cells, then by definition, eyes could not form before there were multicellular animals. ■But many protists (animallike, plantlike, or funguslike unicellular anisms that require a waterbased environment) can detect light by using aggregations of pigment molecules, and they use this information to modify their metabolic activity or motility (the ability to move spontaneously and independently). ■One of the familiar living examples, probably known to anyone who has taken a biology class, is the aquatic protozoan Euglena, which has an eyespot near its motile fIagellum (hairlike structure). ■Some living protists are very like their ancestral forms embedded in ancient sedimentary rocks, and this similarity suggests that the ability to detect light and modify behavior in response to light has been around for a very long time. ■Animals arose from one of such unicellular creatures, perhaps from one already specialized for a primitive kind of vision. 13. Look at the four squares [■] that indicate where the following sentence could be added to the passage. Molaria spinifera and H. Optata, both of which lived in water levels beyond the reach of light, fit into this category. 14. Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points. Drag your choices to the spaces where they belong. To review the passage, click on View Text. Answer Choices  The ability of some unicellular anisms to detect light and change their behavior accordingly suggests that eyes did not originate with multicellular animals.  The earliest eyes apparently contained molecules that were capable of forming and focusing images.  Too few fossils from the Precambrian have been found to determine which if any Precambrian anisms had eyes.  Evidence from the Burgess Shale suggests that eyes of some early animals were similar to the eyes of living crustaceans.  Fossil evidence suggests that anisms in the Burgess Shale with faceted eyes developed later than anisms in the Burgess Shale with n onfaceted eyes.  The large size and possible plexity of the eyes of some anisms in the Burgess Shale suggest that their eyes were not the first eyes. TPO362 The origin of Earth’s atmosphere In order to understand the origin of Earth39。 s atmosphere, we must go back to the earliest days of the solar system, before the plas themselves were formed from a disk of rocky material spinning around the young Sun. This material gradually coalesced into lumps called plaesimals as gravity and chance smashed smaller pieces together, a chaotic and violent process that became more so as plaesimals grew in size and gravitational pull. Within each orbit, collisions between plaesimals generated immense heat and energy. How violent these processes were is suggested by the odd tilt and spin of many of the plas, which indicate that each of the plas was, like a billiard ball, struck at some stage by another large body of some kind. Visual evidence of these processes can be seen by looking at the Moon. Because the Moon has no atmosphere, its surface is not subject to erosion, so it retains the marks of its early history. Its face is deeply scarred by millions of meteoric impacts, as you can see on a clear night with a pair of binoculars. The early Earth did not have much of an atmosphere. Before it grew to full size, its gravitational pull was insufficient to prevent gases from drifting off into space, while the solar wind (the great stream of atomic particles emitted from the Sun) had already driven away much of the gaseous material from the inner orbits of the solar system. So we must imagine the early Earth as a mixture of rocky materials, metals, and trapped gases, subject to constant bombardment by smaller plaesimals and without much of an atmosphere. As it began to reach full size, Earth heated up, partly because of collisions with other plaesimals and partly because of increasing internal pressures as it grew in size. In addition, the early Earth contained abundant radioactive materials, also a source of heat. As Earth heated up, its interior melted. Within the molten interior, under the influence of gravity, different elements were sorted out by density. By about 40 million years after the formation of the solar system, most of the heavier metallic elements in the early Earth, such as iron and nickel, had sunk through the hot sludge to the center giving Earth a core dominated by iron. This metallic core gives Earth its characteristic magic field, which has played an extremely important role in the history of our pla. As heavy materials headed for the center of Earth, lighter silicates (such as the mineral quartz) drifted upward. The denser silicates formed Earth39。 s mantle, a region almost 3,000 kilometers thick between the core and th。