A comparison/analysis of the evolution function and structure of the hearing appartii of reptilians versus mammalians

A comparison/analysis of the evolution function and structure of the hearing appartii of reptilians versus mammalians

One of the greatest stepping stones in the evolutionary scene was the development of mammals from the ancestral reptile.While this involved a number of changes,a major breakthrough in this respect was the development of acute hearing and language in mammals In the context of the hearing apparti in particular there are an increasing number of studies which indicate that the modification of the jaw bones (in reptiles) led to the formation of the middle ear bones in mammals as we shall see later in this essay. However the quote from BBC is indeed an accurate summation of the evolutionary scene.Any comparison of the evolutionary course of the hearing apparti in reptiles and mammals would be incomplete without an overall understanding of what the auditory(hearing) apparatus comprises and its role in perception of sound. Broadly speaking hearing is a result of two linked process. the physical apparatus (ear) which gathers and modulates sound waves and the neurological transmission and processing of the sound waves to a form recognizable to the species.In mammals, the best documented and most extensively studied auditory pathway is seen in man. The human ear consists of three parts – the outer, middle, and inner ear. The outer ear consists of the pinna and the external auditory canal which primarily serve to gather sound waves. These waves enter the middle ear comprising the tympanic membrane and the three auditory bones. The vibrations are transferred down the ossicles after amplification by the ossicles to the oval window membrane which divides the middle ear and the inner ear. It is the inner ear which houses the main organ responsible for hearing, i.e the cochlea. The cochlea is a fluid-filled organ comprising the basilar membrane and the organ of Corti. Sound waves, once in the cochlea, travel across the length of the basilar membrane which lodges the organ of Corti containing sensory cells, that transform these vibrations into the neural code for sound processing by the brain. (Yost 1994)This is a brief overview of the physical auditory apparatus in the modern mammal. While most terrestrial mammals exhibit similar acoustic structure, there are slight modification in aquatic mammals (cetaceans) and bats.
The main difference lies in the absence of the external ear or pinna in cetaceans. In cetaceans, sound waves from the environment are transmitted to the middle ear through the lower jaw. Along with fats that align the lower jaw a thin bony area called the pan bone is instrumental in conducting sound to the tympanic membrane of the middle ear.
The inner ear of cetaceans however is functionally similar to terrestrial mammals with differences primarily in the number of nerve cells, the size of the basilar membrane, and the support of the basilar membrane (Wartzok,1999 pg 117)

This difference between aquatic and non aquatic mammals becomes more interesting in the context of the evolutionary process. It has been established that the equilibrial organ of the primitive bony fish, was passed to subsequent classes which evolved their own auditory systems based on the balance organ inherited from fish. While amphibians independently evolved an an apparatus combining the eardrum and ossicle, reptiles evolution comprised a flexible basilar membrane for the receptor cells. (Peck 1994, pg 291)The external ears were a mammalian addition which as mentioned previously is lacking in aquatic mammals. The aquatic mammals hence demonstrate a link between modern mammals like man and reptiles in their auditory apparatus and hearing mechanisms.

1.2 Evolution of the hearing region

The course of evolution is best studied by means of fossil remains of organisms. Structures which

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