[10/16/08 Correction appended: see end of post]
When our ancestors moved ashore some 360 million years ago, they underwent a lot of changes as they evolved from ocean-swimming fish to land-walking tetrapods. For one thing, they needed feet instead of fins. Paleontologists have discovered a series of fossils that document the early evolution of limb bones in our aquatic ancestors, showing how long bones first evolved, then parts of the wrist and digit-like bones, and finally full-blown feet. But lots of other changes happened at the same time, producing traits in tetrapods not found in other animals. Tetrapods, for example, are good at hearing airborne sounds, thanks to small bones that can vibrate in their ears. The earliest of those bones to have evolved was the stapes. But the stapes did not come from nowhere.
New clues to the origin of our ears were published today in the journal Nature. They come from a fossil known as Tiktaalik, a 370-million-year old fish with a lot of tetrapod features, such as neck and a very leg-like fin. (See this Loom post on Tiktaalik for details.) Tiktaalik's discoverers first published a description of the beast back in 2006, but there was only space for a tiny portion of the details they uncovered in its skeleton. Today they report on Tiktaalik's skull.
In some respects, Tiktaalik's skull was still fish-like. It had a hinge down the middle that allowed the front and back ends to bend. Later in the evolution of tetrapods, that hinge hardened, making the skull stiffer. But other parts of the skull had already evolved to be more like our own. For example, in our close aquatic relatives, there's no neck to separate the head and shoulders. Instead, there are a series of bones. One, called the operculum, covers the gills. Between the operculum and the braincase is a boomerang-shaped bone called the hyomandibula. Some of the muscles attached to it helped fishes pump their gills, and it also buttressed the bones of the skull as they opened and closed when the fish fed. In Tiktaalik, the operculum is gone. The hyomandibula, as a result, lost its connection to the shoulder region.
These changes suggest that Tiktaalik could move its head more, because it was freed from the shoulders. It had also lost the ability to pump a lot of water through its gills. By this point in evolution, the ancestors of tetrapods were relying less on gills for getting oxygen. Living in shallow coastal waters, Tiktaalik may have used its powerful front limbs to do push-ups in order to breathe air instead.
And once the hyomandibula was liberated from the shoulders, it could evolve to specialize for new roles. As other fossils reveal, the hyomandibula would evolve in a small bone that came into contact with the skull, able to transmit vibrations from the air. In other words, it became the stapes. A diagram below shows some stages in this transition.
What's particularly cool about the evolution of our ear is that it was assembled over hundreds of millions of years from other parts. Tiktaalik helps reveal how the stapes evolved some 370 million years ago, but there are other bones in the ear--the incus and malleus--that transmit vibrations in our heads. They only evolved 150 million years later from some bones in the back of the jaw. They too were liberated from old jobs, and free to take on new ones. The ear was not an overnight invention, but the product of an inconceivably long evolutionary tinkering.
Source: Jason Downes et al, "The cranial endoskeleton of Tiktaalik roseae." Nature doi:10.1038/nature07189
Illustration: Kalliopi Monoyios
Ear image: Wikipedia
Correction: I've updated this post to correct some errors about the operculum (gill covering) and hyomandibula. Apologies to the anatomists!