CHAPTER 10:: EARS

- Reichert discovered that two of the bones in our middle ear are the same bones that are parts of the jaw in reptiles. He learned this by tracing the resulting structures during development of embryos. He tracked what happened to those gill arches from the earliest development all the way through a completed organism. There was no mistake, what becomes a jawbone in a reptile becomes the middle ear bones in a mammal. They were the same. But without an evolutionary framework, how can two bones in two widely different organisms be ‘the same’.

- When paleontologists got involved with this line of research (1910-1913) they provided additional evidence. When you look at the very earliest of the mammal-like reptiles, you find one middle ear bone and a jaw composed of several different bones. But as you move upward through time, following the line that started with the mammal-like reptiles and subsequently as you move towards the more mammal-like organisms, you can actually watch the bones that become the middle ear reduce in size and change position. The jawbones had become middle ear bones.

- One of the effects of having a high blood alcohol level is that the inner ear system goes off balance and you stagger around for a day or so. This is because the alcohol from your blood stream goes into your inner ear canals and displaces the fluid that is used in your balance system. That movement of the fluid as it is displaced is sensed by the nerve cells and is reported to the brain as movement when you aren’t moving. The reverse happens the next morning. Your liver has removed most of the alcohol from the blood, and the alcohol in your inner ear slowly diffuses back into the blood stream. The inner ear fluid moves back to where it’s supposed to be and your ear reports that as movement too.

- This inner ear system is directly descended from sharks. A neuromast organ is a tiny little system that sharks use to detect movement of the water around them. Those neruomast organs are similar to the system in your inner ear. The same jelly-like substance in a sac with sensory hairs stuck into it. As the sac is compressed or moves from side to side by ocean currents (or you bending over), the fluid moves the sensory hairs and the shark can sense a fish and you can sense the change in your orientation.

- All this goes back to the genes. This time Pax2 controls the development of the inner ear. A mutation here results in a badly formed or missing inner ear.