Chapter 10: Ears

Ears are made up of three sections: outer, or external, the middle, and the inner. The external ear is the newest addition, with the pinna (external ear flap) and is only present in mammals. Mammals also have three middle ear bones, however, reptiles and amphibians have only two, while fish have none! This fact surprised me because I didn't know that our ear bones were so special. A scientist by the name of Reichert discovered that parts of the ears of mammals are the same as parts of jaws of reptiles.(This was extremely revolutinary for his time since this was before Darwin's proposal of evolution). Later on, a scientest named Gaupp used Reichert's discovery to say the three middle ear bones reveal a tie between reptiles and mammals; if indeed the case, the fossil record should show bones shifting from jaw to ear during the origin of mammals...which was eventually confirmed! The origin of mammals include new ways of hearing by repurposing exisiting bones found in reptiles! The middle ear contains the records of two important subjects: one is the origin of our stapes and transformation of a jaw support bone to an ear bone, beginning when fish started to walk on land, and two, the bones at the back of the reptile jaw became our malleus and incus. Shubin helped me understand the inner ear and its gel by comparing it to a snow globe. Once I understood this concept, I also understod the inner ear's basic chain of events: gel inside swishes, hair projections on nerves bend, and an impulse is sent to our brains. This process is made more sensitive with tiny rock-like structure on top of the membrane that allows the gel to move more inside our inner ear. Our inner ear correlates with the eye to assess position and acceleration. Shubin said the best way to understand this is by obstructing it...by sharing what happens in the inner ear when a person is drunk. I found it funny when he said we can thank these occurences because of our shared history with sharks and fish, stating, 'the spinning drunk and fish's water-sensing organs have common genes.' He also stated that mammals obtained a better sense of hearing because their inner ear is developed in a different way compared to other animals. Scientists discovered Pax 2, which is a gene active in the ear region that starts a chain reaction of gene activity that leads to the development of the inner ear. Pax 2 reminded me of Pax 6 from the previous chapter and I was glad to know that they seem to have a link other than their similar names. The box jellyfish (which has eyes!!!) shows that although it does not have either of these genes, it has a gene that 'is a mosaic' to the structure of both Pax 2 and Pax 6...in other words, this jellyfish had a primitive version of both of these genes!