Paleontologists have long struggled to explain how vertebrates evolved legs. One popular theory is that ancient fish, stranded when ponds dried up, used muscular fins to drag themselves to new bodies of water. The fish that were able to cover the most ground were more likely to survive, causing true legs to evolve. In other words, this theory holds that fish began crossing dry land before they began evolving legs.

Newly discovered fossils of Acanthostega, an early air-breathing fishlike creature, however suggest that legs began evolving in vertebrates that were still completely aquatic. Acanthostega's limbs would have been unable to support its weight on land, looking something like paddles for swimming. And although it had lungs as well as gills, its ribs were too short to prevent the chest cavity's collapse once out of water.

Acanthostega's limbs may have evolved simply to allow the animal to raise its head above oxygen-poor water to breathe. A fossilized upper arm bone from a similar species bolsters this idea. The bone attached to the shoulder with a hingelike joint, as opposed to the ball-and-socket shoulder joint shared by modern land vertebrates. This arrangement would not have permitted walking, but it would have enabled a simple pushup for a gulp of air.



Which of the following most accurately describes the function of the first paragraph in the context of the passage?


To describe a version of a theory that is modified but largely preserved in later paragraphs

To refute a popular theory that is in competition with a theory that is described, though not argued for, in later paragraphs

To introduce a theory for which evidence is presented in later paragraphs

To describe a theory that is called into question in later paragraphs

To introduce the type of creature on which the argument detailed in later paragraphs is based

考题讲解

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正确答案为 C。因为在第一段,作者描述了一个流行的理论,即古代鱼类在池塘干涸时使用肌肉鳍来拖拽自己进入新的水域。在之后的段落中,作者提出新的证据反对这种理论,特别是针对 Acanthostega 这种早期可以呼吸空气的鱼类生物,它们的腿无法在陆地上承受重量,这个证据表明发育腿的鱼类仍然位于完全水生的环境下。因此,正确的答案应该选择 C,因为在第一段中,作者描述了一个理论,并在随后的段落中提供证据进一步证明其正确性。

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