首页
外语
计算机
考研
公务员
职业资格
财经
工程
司法
医学
专升本
自考
实用职业技能
登录
外语
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 f
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 f
admin
2013-01-12
38
问题
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.
【66】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 ancient conditions in the laboratory. If plausible gases are mixed in a flask with water, and energy is added by an electric discharge (simulated lightning), organic sub-stances 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.【67】
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.【68】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: hat 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.【69】
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, lawless fishes with fins, gills, and fish-like muscle patterns--found in China in 1999. Vertebrates appear abundantly in fossil beds between 300 and 400 million years ago.【70】
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. 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.
F. 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.
选项
答案
E
解析
本题空格出现在段末。上文提到,接下来的进化过程是复制分子(或称为基因)自然淘汰的继续。空格处内容应弥补这个信息空白。选项中只有E提到, 30亿年的时间里,动物产生了有椎骨的身体,恰当地衔接了上下文。
转载请注明原文地址:https://kaotiyun.com/show/vk9O777K
0
考博英语
相关试题推荐
Atradegroupforliquorretailersputoutapressreleasewithanalarmingheadline;"MillionsofKidsBuyInternetAlcohol,La
Atradegroupforliquorretailersputoutapressreleasewithanalarmingheadline;"MillionsofKidsBuyInternetAlcohol,La
Atradegroupforliquorretailersputoutapressreleasewithanalarmingheadline;"MillionsofKidsBuyInternetAlcohol,La
LarryGriffinwaswokenatthreethatmorningtobetoldthathislast-minute______againsthisdeathsentenceforthemurderof
AnotherexampleoftheexerciseofpowerbyCongresswastheactionittookduringtheReconstructionPeriodaftertheCivilWar
AnotherexampleoftheexerciseofpowerbyCongresswastheactionittookduringtheReconstructionPeriodaftertheCivilWar
Buildingthisroadwill______theconstructionoftenbridges,thenthetotalcostreaches1millionUSDollars.
A、becauseofB、sinceC、becauseD、thankstoA本题考核的知识点是连词。分析上下旬逻辑关系,逻辑是一致的。四个选项都有“因为”的意思,但侧重点不同。A项侧重表直接原因;B项侧重表“作为…的结果”,后面接句子;C项必须
used本题空格处的句子是一个定语从句,是说罐子是用来喝水的。短语beusedfor意为“用来做……”。
随机试题
患者中年女性,无明显诱因出现手足发麻,关节肿痛半年余。开始为手指小关节疼痛,后出现其他关节疼痛,呈对称性,遇寒或晨起时关节发硬,活动后减轻。其最有意义的检查是
小儿较成人脉象
使细菌具有侵袭力的结构成分是
A.砒霜B.洋地黄毒苷C.丁丙诺啡D.地西泮属于毒性西药品种的是
所谓的“世界末日”——地球会彻底毁灭,这并非完全源于玛雅预言,而是综合了古代和现代多种文化和宗教传说而产生的。墨西哥碑铭专家欧布莱孔.克莱林曾表示,玛雅人从未预测“世界末日”,他们留下的文字记载有10000多条,其中只有一条谈到2012年,而且
(2012下集管)项目收尾包括______。
在页式存储管理中,为进行地址转换工作,系统提供一对硬件寄存器,它们是
将考生文件夹下VOTUNA文件夹中的BOYABLE.DOC文件复制到同一文件夹下,并命名为SYAD.DOC。
Readthearticlebelowaboutsellingsandwiches.Choosethecorrectwordtofilleachgap,fromA,BorC.
WhatisPET?TheCambridgePreliminary(预备的)EnglishTest(PET)isthesecondleveloftheCambridgeexamsinEnglishforSpe
最新回复
(
0
)