Part B

　　Directions:

　　In the following article, some sentences have been removed. For Questions 21-25, choose the most suitable one from the

　　list A-G to fit into each of the numbered blank. There is one extra choice that does not fit in any of the gaps. Mark your answers on ANSWER SHEET 1. (10 points)

　　Between 5,000 million and 4,000 million years ago the Earth was formed. By 3,000 million years ago life had arisen and we have fossils of microscopic bacteria like creatures to prove it. (21) ————. Nobody knows what happened, but theorists agree that the key was the spontaneous arising of self replicating entities, i.e. something equivalent to “genes” in the general sense.

　　The atmosphere of the early Earth probably contained gases still abundant today on other planets in the solar system. Chemists have experimentally reconstructed these ancientconditions in the laboratory. If plausible gases are mixed in a flask with water, and energy is added by an electric discharge (simulated lightning), organic substances are spontaneously synthesized. These include the building blocks of RNA and DNA. It seems probable that something like this happened on the early Earth. Consequently, the sea would have become a “soup” of prebiological organic compounds. (22)————.

　　Today the most famous self replicating molecule is DNA (deoxyribonucleic acid), but it is widely thought that DNA itself

　　could not have been present at the origin of life because its replication is too dependent on support from specialized machinery, which could not have been available before evolution itself began. DNA has been described as a “high tech” molecule which probably arose some time after the origin of life itself. Perhaps the related molecule RNA, which still plays various vital roles in living cells, was the original self replicating molecule. Or perhaps the primordial replicator was a different kind of molecule altogether. (23) ————. Variants that were particularly good at replication would automatically have come to predominate in the primeval soup. Varieties that did not replicate, or that did so inaccurately, would have become relatively less numerous. This led to ever increasing efficiency among replicating molecules.

　　As the competition between replicating molecules warmed up, success must have gone to the ones that happened to hit

　　upon special tricks or devices for their own self preservation and their own rapid replication. The rest of evolution may be

　　regarded as a continuation of the natural selection of replicator molecules, now called genes, by virtue of their capacity to

　　build for themselves efficient devices (cells and multicellular bodies) for their own preservation and reproduction.

　　(24)————.

　　Fossils were not laid down on more than a small scale until the Cambrian era, nearly 600 million years ago. The first vertebrates may date back 530 million years, according to fossil evidence—primitive, jawless fishes with fins, gills, and fishlike muscle patterns—found in China in 1999. Vertebrates appear abundantly in fossil beds between 300 and 400 million

　　years ago. (25)————. Mammals and, later, birds, arose from two different branches of reptiles. The rapid divergence of mammals into the rich variety of types that we see today, from opossums to elephants, from anteaters to monkeys, seems to have been unleashed into the vacuum left by the catastrophic extinction of the dinosaurs, 65 million years ago.

　　\[A\] Among vertebrates, the land was first colonized by lobe finned and lung bearing fish about 250 million years ago,

　　then by amphibians and, in more thoroughgoing fashion, by various kinds of animals that we loosely lump together as “reptiles”.

　　\[B\] Once self replicating molecules had been formed by chance, something like Darwinian natural selection could have

　　begun: variation would have come into the population because of random errors in copying.

　　\[C\] It is not enough, of course, that organic molecules appeared in the primeval soup. The crucial step, as noted above, was the origin of self replicating molecules, molecules capable of copying themselves.

　　\[D\] Although we naturally emphasize the evolution of our own kind—the vertebrates, the mammals, and the primates—these constitute only a small branch of the great tree of life.

　　\[E\]When the environment changes, or when organisms move to a different environment, different variations are selected, leading eventually to different species. Harmful mutations usually die out quickly, so they don t interfere with the process

　　of beneficial mutations accumulating.

　　\[F\] Three thousand million years is a long time, and it seems to have been long enough to have produced such astonishingly complex contrivances as the vertebrate body and the insect body.

　　\[G\]Some time between these two dates—independent molecular evidence suggests about 4,000 million years ago—that

　　mysterious event, the origin of life, must have occurred.[748 words]

　　核心詞匯

　　by/in virtue of 憑借(某種手段);由于，因?yàn)?例：He was exempt from charges by virtue of his youth/of being so young.他因年幼而獲得免費(fèi). / She became a British citizen by virtue of her marriage.她借助婚姻成了英國(guó)公民。

　　extinction n. [U] *①(物種)滅絕，絕種 例：be threatened with extinction瀕臨滅絕 ②(信仰、生活方式等)遭毀滅，消失，消亡

　　hit on/upon ① (經(jīng)長(zhǎng)時(shí)間考慮后)突然想出(好主意) 例：In this way we hit upon over 20 useful methods of work.

　　就這樣我們想出了20 多種有用的工作方法。 *② 偶然發(fā)現(xiàn)(某物) 例： At last she hit upon a room that suited them nicely.最后她找到了一間適合他們使用的房間。

　　in more thoroughgoing fashion 以更徹底的方式;in a... fashion以„„方式 例：Leave the building in an orderly fashion.

　　要有秩序地走出樓去。 interfere with *① 妨礙，打擾 例：Don t allow pleasure to interfere with duty.不要讓娛樂(lè)妨礙了職責(zé)。 ② 干擾(廣播或電視播送) lay down ① 放下(武器等) 例：The general told the troops to lay down their arms.將軍命令部隊(duì)放下武器。 ② [常用被動(dòng)態(tài)](正式地或堅(jiān)決地)闡述，聲明，規(guī)定 例：It is laid down that all applicants must sit a written exam.根據(jù)規(guī)定, 申請(qǐng)者一律需經(jīng)筆試。 ③ 開(kāi)始建造;開(kāi)始做(奠基性的工作) 例：Crick and Watson laid down the foundations of modern genetic research.克里克和沃森為現(xiàn)代遺傳學(xué)研究奠定了基礎(chǔ)。 *④ (化石、地層等的)沉積形成