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