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  3. 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

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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A
解析 本题空格出现在段中。上文提到脊椎动物的大量出现，下文提到哺乳动物和鸟类从爬行动物的两个分支中衍生出来。哺乳动物的迅速分支，又形成了今天我们见到的丰富的动物类型。上下文之间有时间上的跨越，因此预测空格处应弥补这种信息上的空缺，即承接上文，又能够引出下文。故选A。
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