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.
The passage states that Acanthostega
was an ancestor of land-dwelling mammals
had a chest cavity that would have collapsed on dry land
had a ball-and-socket shoulder joint
was the first vertebrate with true legs
used its limbs only as paddles for swimming