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 bacterialike creatures to prove it. (21) ————. Nobody knows what happened, but theorists agree that the key was the spontaneous arising of selfreplicating 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 selfreplicating 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 “hightech” 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 selfreplicating 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 everincreasing 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 selfpreservation 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 lobefinned and lungbearing 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 selfreplicating 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 selfreplicating 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］

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