On the Origin of Orifices: Ears

Ears aren’t the most beautiful part of the human body. Even if you’re lucky enough not to have a pair that stick out at 90 degrees or spontaneously sprout tufts of hair, many people still feel that the only way to improve their appearance is to punch holes in them and add shiny bits of metal. 

Still, those flappy bits of cartilage should not write off the amazing structures that are hidden inside our skulls. The mammalian ear is very sophisticated:  sound collected by the outer ear is channelled into the middle ear, where it is amplified by the ear drums. The sound then passes via a group of small bones into the fluid-filled cochlea of the inner ear, which is lined with sensitive hair cells. When these are disturbed by sound waves, they move and fire nerves at their bases, sending off information to be processed by the brain. Other structures in the inner ear also help us with balance (though they still haven’t evolved to cope with fairground rides and student drinking games).

But this complexity has not always been there. How did we end up with holes in the sides of our heads, anyway? And where did all those complicated bits inside come from? Well, listen carefully. Or read carefully. Eyes are probably the more useful organ for understanding this post: let your ears relax for a bit and put on some pretty music. That’s right.

You might notice that you can’t point out the ears on a fish: the inner ear is the only part of the trio found in most modern fishes- reflecting the situation in early vertebrates.

'I never wanted your stoopid ears anyway.'

 Sound travels well in water, but as fish have a similar density to water, their entire bodies and their surroundings are equally affected by sound waves, making them very difficult to detect. Tiny hair cells like those in our cochlea are found down either side of fishy bodies and within the inner ear, and first appeared at least 440 million years ago. However, without altering the relative speed of sound waves, these hairs would not have been any good for sound detection. Modern fishes have got around this using a collection of very dense otolith bones in their inner ears. Sound waves move these bones more slowly than the rest of the fish’s body, causing the inner ear’s sensory hairs to distort and fire their nerves. Some species can improve their sensitivity further by connecting their ears to their swim bladders, which are filled with gas and thus slow down the sound waves that pass through them, allowing detection.

Now, you might remember from a few posts ago that early vertebrates started doing interesting things with the structure of their gills. The first pair of gill bars changed to form the jaw, and the second became the jaw-supporting hyomandibular bone; but such a major alteration in the gills’ internal anatomy was bound to affect its other features, too.  And it did: the first pair of gill openings that once served to flush used water out into the environment became squashed and shrunken, going from large, muscular flaps to a small, pretty useless hole leading into the mouth on either side of the head, called the spiracle. Useless yet harmless structures like this tend to have one of two fates in evolution. They either slowly disappear over evolutionary time- like a hamster’s tail- or undergo chance modifications that make them worth keeping around after all. We might never have known the spiracle had ever existed if it hadn’t been recruited for a new role in our ancestors!

Some fish probably started using the spiracle as a novel way of sucking in water to breathe, as an alternative to the mouth when at rest. Modern sharks have developed a similar “breathing hole” that they use when chilling on the sea floor, to avoid sucking gravel into their delicate gill system (ouch). Fossilised fishy ancestors of the first tetrapods (all four-limbed vertebrates, from frogs to humans to whales) show an enlarged spiracle for this purpose, allowed by the shrinkage of the hyomandibular bone. This bone later went on to shrink even more, becoming the stapes (or “stirrup”) of the middle ear in tetrapods. Here, it took on the function of relaying vibrations from the (also new) eardrum to the liquidy depths of the middle ear, making them louder in the process. This new structure was necessary when our ancestors came onto land, and found themselves deaf: sound has a harder time travelling through air, and the weedy little vibrations on land just weren’t enough to be picked up by their inner ears without amplification. So the spiracle switched roles once again, and became a sound-window to the outside world.

I propose an underwater kingdom for the elderly!

This bony amplification system has been honed even further in the mammals, and again, its evolution was tightly linked to that of the jaw. Reptiles and amphibians have multiple bones joined together to make up their lower jaws: the dentary, the quadrate and the articular. But this is rather like making a scissor blade out of three separate pieces glued together instead of a single block of metal: it’s never going to be quite as strong. In search of a more powerful bite, the ancestors of mammals evolved a larger dentary bone, which eventually became the sole bone of the lower jaw. Reduced in size and now a little bit useless, the quadrate and articular could easily have disappeared altogether- but like the spiracle, they were saved by recruitment into the ear system, where they teamed up with the stirrup bone. The transition can be seen in fossils of the lineage that led to mammals: at some stages, the quadrate bone appears to have had a dual function, remaining as part of the jaw whilst transmitting vibrations to the inner ear. In their present form, we generally know the articular and quadrate bones as the anvil and the hammer due to their shapes- slightly catchier titles, really. They may seem to have been demoted a little, having gone from being some fairly significant chunks of bone to being the smallest in the (human) body. However, they play an important role in broadening the range of pitches that mammals can detect- allowing whales and bats to use sonar, and humans to appreciate opera and car alarms. Not a bad new job, really. 

So learn to love your ears, because scientists certainly do! The evolution of many aspects of hearing has been mercifully easy to study compared to the other senses, simply because so many bones are involved- and bones are much more likely to turn up in the fossil record than, say, eyeballs. Given that the jaw and the ear are so closely linked, it shouldn't be too difficult to come up with a clever bit of backchat to anyone who admonishes you for using your mouth more than your ears, but I'll let you figure that one out. That’s enough ears for now: in the next Origin of Orifices, we’re taking a trip to the other end of the digestive system. No giggling!

Image credits: Clownfish- http://www.flickr.com/photos/tambako/4188752328/
Old lady: http://www.flickr.com/photos/louisa_catlover/5581012353/