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Introduction
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 cochlea exists the organ of Corti, which is composed of hair cells that are responsible for translating the vibrations it receives from surrounding ducts into electrical impulses that are sent to the brain to process sound. This spiral-shaped cochlea is estimated to have originated during the Jurassic Period, around 150-200 million years ago. Further, the auditory innervation of the spiral-shaped cochlea traces back to the Cretaceous period, around 66-150 million years ago. The evolution of the human cochlea is a major area of scientific interest because of its favourable representation in the fossil record. The last century has consisted of many scientists such as evolutionary biologists and paleontologists striving 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 its body. In more recent times, technologies such as micro-CT scanning are 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.

Comparative Anatomy of the Cochlea
Birds and mammals have similar auditory systems, including cochlea, while the rest of the animal kingdom does not. Instead, reptiles, amphibians, and fish hear with simpler auditory organs. The cochlea allows for higher sensitivity for higher frequencies, thus those lacking it hear lower frequencies.

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Reptiles
Reptiles have a membranous opening on the vestibuli (a lymph-filled canal in the bony canal of the cochlea that receives vibrations from the stapes) called the oval window on which sound is transmitted by the stapes (one of three small bones in the middle ear that conduct vibrations received by the inner ear).

Birds, Mammals, and Crocodilians
Whereas reptiles have the basilar papilla (composed of epithelial cells), basilar membrane (which separates the scala media and the scala tympani (liquid-filled tubes) within the cochlea), and lagena, mammals, birds, and crocodilians have the organ of Corti (a strip of epithelial cells that allows for the conversion of auditory signals into nerve impulses), scala vestibuli/tympani (conducts sound vibrations to the cochlear duct), and cochlear duct (an endolymph-filled cavity within the cochlea), respectively. Because of the size of the head, the mammalian cochlea is extended even more, forming a spiral.

Amphibians
Amphibians and reptiles have very similar inner ear structures. However, Amphibians lack the basilar papilla and instead have the papilla amphibiorum (homologous to the organ of Corti in mammals).

Fish
Interestingly, fish, while having a lagena (corresponds to the cochlear duct in mammals), have no use for it for auditory sensations, and rather use the cluster called macula neglecta (arrays of sensory hair hairs scattered amongst supporting cells).

Neanderthals to Modern Humans
The size of cochlea has been measured throughout its evolution based on the fossil record. In one study, the basal turn of the cochlea was measured, and it was hypothesized that cochlear size correlates with body mass. The size of the basal turn of the cochlea was not different in Neanderthals and Holocene humans, however it became larger in early modern humans and Upper Paleolithic humans. Furthermore, the position and orientation of the cochlea is similar between Neanderthals and Holocene humans, relative to plane of the lateral canal, whereas early modern and upper Paleolithic humans have a more superiorly placed cochlea than Holocene humans. When comparing hominins of the Middle Pleistocene and Neanderthals and Holocene humans, the apex of the cochlea faces more inferiorly in the hominins than the latter two groups. Finally, the cochlea of European middle Pleistocene hominins faces more inferiorly than Neanderthals, modern humans, and Homo erectus. Human beings, along with Apes, are the only mammals that do not have high frequency (>32kHz) hearing. Humans have long cochlea, but the width of their basilar membrane is relatively short, resulting in a comparatively lesser ability to sense high frequency auditory signals. The human cochlea has approximately 2.5 turns around the modiolus (the axis). In addition, humans are uniquely equipped to be able to perceive auditory signals that displace the eardrum by a mere picometre. Furthermore, human beings have the ability to discriminate sounds with great efficiency at varying frequencies.



The Ear
Because of its prominence and preserved state in the fossil record, until recently, the ear had been used to determine phylogeny. Now, the ear region is used to note the changes that occurred during and as a result of the water-to-land transition. The ear itself contains different portions, including the outer ear, the middle ear, and the inner ear [14]. There are many different mechanisms at play when animals sense auditory signals, including the tympanic membrane (ear drum), the Organ of Corti (spiral organ), and the cochlea itself. The presence of sound waves leads to cochlear fluid displacement, creating electrical signals.

Evolutionary Perspective
The cochlea is the tri-chambered auditory detection portion of the ear, consisting of the scala media, the scala tympani, and the scala vestibuli. Regarding mammals, placental and marsupial cochleae have similar cochlear responses to auditory stimulation as well as DC resting potentials. This leads to the investigation of the relationship between these therian mammals and researching their ancestral species to trace the origin of the cochlea.



This spiral-shaped cochlea that is in both marsupial and placental mammals is traced back to approximately 140-170 million years ago. The development of the basilar papilla (the auditory organ that coincides with the Organ of Corti in mammals) happened at the same time as the water-to-land transition of vertebrates, approximately 380 million years ago. The actual coiling or spiral nature of the cochlea occurred to save space inside the skull. The more coiled and the longer the cochlea, the higher the resolution of sound frequencies. The oldest of the mammalian cochleae were approximately 2 mm in length. In egg-laying mammals, or monotremes, the cochlea stayed short and non-spiraled.

The earliest evidence available for primates depicts a short cochlea with prominent laminae, meaning they had good high-frequency sensitivity as opposed to low-frequency sensitivity. After this, over a period of around 60 million years, evidence suggests that primates developed longer cochleae and less prominent laminae, which means that they had an increase in low-frequency sensitivity and a slight decrease in high-frequency sensitivity. By the early Miocene period, the cycle of the elongation of the cochleae and the deterioration of the laminae was completed. The primates had a high sensitivity to high frequencies and a poor sensitivity to low frequencies. Evidence shows that primates have had an increasing cochlear volume to body mass ratio over time. These changes in the cochlear labyrinth volume negatively affect the highest and lowest audible frequencies, causing a downward shift. Non-primates appear to have smaller cochlear labyrinth volumes overall when compared to primates. Some evidence also suggests that selective forces for the larger cochlear labyrinth may have started after the basal primate node. Mammals are the subject of a substantial amount of research not only because of the potential knowledge to be gained regarding humans, but also because of their rich and abundant representation in the fossil record. Some new evidence suggests that the spiral shape of the cochlea evolved later on in the evolutionary pathway of mammals than previously expected.

The Evolution of Prestins
Parallel to the evolution of the cochlear makeup, prestins evolved in therian mammals. Prestins are located in the outer hair cells of mammalian cochlea and are considered motor proteins. They are essential to the hearing of all vertebrates, including fish, and are thought to have evolved as transporters first. A high concentration of prestins are found solely in the mammalian lateral membranes of the outer hair cells. This high concentration is not found in inner hair cells, and is also lacking in all hair cell types of non-mammals. Prestin also has a role in motility, which evolved a greater importance on motor function in land vertebrates, but this developed vastly differently in different lineages. In certain non-mammals and monotreme mammals, prestins function as both transporters and motors, but the intense development of robust motor dynamics, as seen in therian mammals, failed to evolve. It is hypothesized that this intense motor system is significant to the therian cochlea at high frequencies because of the distinctive cellular composition of the organ of Corti that allows the prestins to intensify movements of the whole structure. It has been suggested that prior to the evolution of therians, prestins of other mammals (such as monotremes) lacked the specialization of a motor system at high frequencies. Modern ultra-sound echolocating species such as bats and toothed whales contain significantly evolved and specific prestins, and these prestins show many identical sequence alterations over time. Unusually, the sequences evolved independent from each other and did not occur during the same time periods. Furthermore, the evolution of neurotransmitter receptor systems (acetylcholine) that regulate the motor feedback of the outer hair cells coincides with prestin evolution in therians. This suggests that there was a parallel evolution of a control system and a motor system in the inner ear of mammals.

Homoplasies (Convergent Evolution)
The auditory abilities in amniotes do not increase in effectiveness successively, rather they came about through parallel evolution. Likewise, in all animals, including mammals (and humans), it is important to note that similar phenotypes are sometimes a result of convergent evolution or homoplasy. Specifically, evidence from fossils depict homoplasies for the detachment of the ear from the jaw. Furthermore, it is apparent that the land-based eardrum, or tympanic membrane, evolved convergently in multiple different settings as opposed to being a defining morphology.