Chapter 10: Ears

Ameen Farwana

Chapter 10 Inner Fish Blog

1)      There are three parts to the ear; the inner ear, middle ear, and outer ear. The pinna, or the flap of the external ear, is a body part only found in mammals.

2)                  The discovery that the ear bones of mammals may have some correlation to the jaw bones of reptiles began with Karl Reichert, a German anatomist. Mammals have three bones in the middle ear, while reptiles and amphibians have only one. Thus, these bones had to come from somewhere. He observed gill arches in several species to see where they ended up within the skull, and discovered that two of the ear bones present in mammals corresponded to bones in the jaws of reptiles. He concluded that “the same gill arch that formed part of the jaw of a reptile formed ear bones in mammals” (Shubin 160). Another German anatomist, Ernst Gaupp, continued this theory, believing that the three middle ear bones showed a tie between reptiles and mammals. The two mammalian middle ear bones that correlated to reptiles, the malleus and the incus, evolved from parts of the reptilian jaw. In the 1840s, new fossil creatures were being discovered in South Africa and Russia; when put together, many of them were described as mammal-like reptiles.

By 1913, embryologists and paleontologists began to work together, and with Gaupp’s theory, began to look at these fossilized skeletons. They discovered that the most reptilian of these skeletons only had a single bone in the middle ear, just like current reptiles do, and a jaw composed of many bones. However, the more mammalian of these skeletons showed the bones at the back of the reptilian jaw getting smaller until they eventually became part of the middle ear of mammals. This proved that the malleus and incus evolved from jawbones.

Similar discoveries were made between humans and sharks. The stapes is a second arch bone in the ear, and corresponds to a bone in sharks and fish known as the hyomandibula. The hyomandibula is a large rod that joins the upper jaw with the braincase. Looking at fossils being traced from sharks to Tiktaalik to amphibians, there is a clear trend of the hyomandibula shrinking in the upper jaw and eventually shifting position to play a role in hearing.

 

3)                  The Pax 2 gene is active in the ear and starts a chain reaction that allows for the inner ear to develop. If a mutation in humans or mice knows this gene out the inner ear cannot properly form. This is a major gene that is essential for proper development.

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3 Comments

  1. Yet another evolutionary trend. We could add a column to the Animal kingdom page of your organization of life and evolutionary trends packet called “gill arches to …”. As you read down the column, it would reveal the formation of ear bones through sharks, amphibians, reptiles and mammals.
    Do all major animal groups share the Pax 2 gene? If yes, what transcription factors are responsible for it’s activation in mammals, that other groups lack?
    The slide show is helpful. You ought to give credit to its source however. ML

    Reply
  2. I think chapter really solidified the versatility of these gill arches, and also how the ancestral jawbone had really impacted development of present organisms.

    Reply
  3. So, this Pax 2 gene regulates the pattern of development of the gill arches? It would be intriguing to explore what genes actually manipulate the ways in which these gill arches develop, since, while resulting structures are homologous, they are very divergent in appearance.
    Arman

    Reply

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