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 way the second paragraph functions in the context of the passage?


It raises a concern to which the next paragraph proposes a solution.

It identifies an accepted method that is questioned in the next paragraph.

It describes an approach that the next paragraph suggests is insufficient.

It offers an explanation that is complicated by information provided in the next paragraph.

It explains the details of a process and thus reveals a problem articulated in the next paragraph.

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