Evolution of the cochlea

The word cochlea /ˈkɒklɪə/ is Latin for “snail, shell or screw” and originates from the Greek word kohlias. The modern definition, the auditory portion of the inner ear, originated in the late 17th century. Within the mammalian cochlea exists the organ of Corti, which contains hair cells that are responsible for translating the vibrations it receives from surrounding fluid-filled ducts into electrical impulses that are sent to the brain to process sound. This spiral-shaped cochlea is estimated to have originated during the early Cretaceous Period, around 120 million years ago. Further, the auditory innervation of the spiral-shaped cochlea also traces back to the Cretaceous period. The evolution of the human cochlea is a major area of scientific interest because of its favourable representation in the fossil record. During the last century, many scientists such as evolutionary biologists and paleontologists strove to develop new methods and techniques to overcome the many obstacles associated with working with ancient, delicate artifacts. In the past, scientists were limited in their ability to fully examine specimens without causing damage to them. In more recent times, technologies such as micro-CT scanning became available. These technologies allow for the visual differentiation between fossilized animal materials and other sedimentary remains. With the use of X-ray technologies, it is possible to ascertain some information about the auditory capabilities of extinct creatures, giving insight to human ancestors as well as their contemporary species.

While the basic structure of the inner ear in lepidosaurs (lizards and snakes), archosaurs (birds and crocodilians) and mammals is similar, and the organs are considered to be homologous, each group has a unique type of auditory organ. The hearing organ arose within the lagenar duct of stem reptiles, lying between the saccular and lagenar epithelia. In lepidosaurs, the hearing organ, the basilar papilla, is generally small, with at most 2000 hair cells, whereas in archosaurs the basilar papilla can be much longer (>10mm in owls) and contain many more hair cells that show two typical size extremes, the short and the tall hair cells. In mammals, the structure is known as the organ of Corti and shows a unique arrangement of hair cells and supporting cells. All mammalian organs of Corti contain a supporting tunnel made up of pillar cells, on the inner side of which there are inner hair cells and outer hair cells on the outer side. The definitive mammalian middle ear and the elongated cochlea allows for better sensitivity for higher frequencies.

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