Comparison of the Structure of the Hearing Appartii of Reptilians versus Mammalians - Essay Example

A comparison/analysis of the evolution, function, and structure of the hearing appartii of reptilians versus mammalians "You can hear echoes of your evolutionary past every time you chew" BBC, Journey of Life 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 fossilize most easily are bones and teeth and hence form the subject of most research work attempting to analyse transitional species from the past. Two established osteological differences between reptiles and mammals in terms of evolution of the auditory apparatus are the occurrence of four lower jaw bones in reptiles (i.e. the dentary, articular, angular, surangular, and coronoid) as opposed to the single bone (the dentary) which forms the lower jaw in mammals and the occurrence of only one middle ear bone (the stapes)in reptiles in contrast to the three bones in mammals (the hammer, anvil, and stapes). Developmental biologists soon hypothesized a potential reason for this evolutionary discrepancy. Based on embryological experimental results it was suggested that two developing bones in the fetus from the head region finally form two bones in the reptilian lower jaw i.e the quadrate and the articular while the corresponding developing bones in the mammalian fetus eventually form the anvil and hammer (malleus and incus) of the mammalian middle ear (Gilbert 1997, pp. 894-896). The migration pattern of the embryological osteocytes (bone cells) hence is clearly indicative of the fact that the two additional ear ossicles in the middle ear in mammals have evolved from the reptilian jawbones. While intermediate fossil forms have so far remained elusive a recent discovery of the fossil Panderichthys where the stirrup bone or stapes appears to be in the middle of moving to a part of the skull where it would become an ear bone is now being studied as a major breakthrough in the course the auditory apparati evolution of reptiles and from reptiles to mammals. (Brown 2006) While it has been established beyond doubt that hearing organs of all vertebrates are homologous and have a common ancestory, there is a wide range of morphological differences between them. It is unlikely that structural diversity should not have some functional consequence or basis. A study of the middle ear (described above) does not reveal much functional difference. However the inner ear and the cochlea in particular do demonstrate significant differences. The main difference lies in the frequency detection sensitivity of the basilar membrane, BM pappilae (hair cells). Most reptililian forms do not show any special frequency selectivity (O Neill. 1995 pp 125) while in mammals there is a high level of frequency selectivity owing to the motor activity of hair cells( Sellick et al. 1982 pp 131). This can be simply put by saying that the mammalian ear is much better at transmitting high frequency sounds than the reptilian ear. Approximate stratigraphic ranges of the various taxa are indicated at the far left (more recent on top). The left column of jawbones shows the view of the left jawbone from the inside of the mouth. The right column is the view of the right jawbone from the right side (outside of the skull). As in Figure 1.4.1, the quadrate (mammalian anvil or incus) is in turquoise, the articular (mammalian hammer or malleus) is in yellow, and the angular (mammalian tympanic annulus) is in pink. For clarity, the teeth are not shown, and the squamosal upper jawbone is omitted (it replaces the quadrate in the mammalian jaw joint, and forms part of the jaw joint in advanced cynodonts and Morganucodon). Q = quadrate, Ar = articular, An = angular, I = incus (anvil), Ma = malleus (hammer), Ty = tympanic annulus, D = dentary. (Reproduced from Kardong 2002, pp. 274, with permission from the publisher, (Copyright 2002 McGraw-Hill) The above diagram (taken from Kadong 2002), provides a good overview of the evolutionary sequence of the jawbones and the ear bones from the primitive reptilian forms to the modern mammalian structures observed now. However, that naturaaly brings us to the question of the functionality of these modifications and the underlying causes for the evolutionary course. 1.3 Funtionality of this region The importance of the middle ear and its role in conducting aerial sound in the hearing process in reptiles has been extensively discussed by many biologists. (Manley 1990; Werner 1972; Werner et al. 1991). Amongst the animals of class reptilia lizards (gekkonomorph lizards in particular) species are a considerably important species for studying auditory structure and function because of their inherent and unique ability of producing audible sound patterns. A common feature in lizard families in terms of the auditory apparatus is that lizard papillae comprise two types of hair cells (Miller 1992). The first of these are hair cells which are thick and innervated by several afferent and efferentnerves. These cells are responsible for transmission and perception of of low frequencies sound waves (below about 1kHz). The other group of hair cells are smaller in structure, completely lack efferent innervation and respond to frequencies higher than1kHz. This morphological separation of the ear into areas responding only to particular frequencies impairs considerably the functionality of the ear in lizards, and makes it less evolved than that observed in mammals. In most mammalian forms there is a gradual transition from small hair cells to intermediate hair cells to long hair cells in the cochlear region and each of them are separated by specialized supporting cells. This provides for a wider range and better selectivity of frequency in mammals (primates and man included). In addition there is no localization of specific hair cells to specific region of the basilar membrane in man. The reason or selection pressure for this evolutionary change from reptiles to man is yet to be established and is presently the subject of much scientific research. A consequence of the evolution of the patter of papillary hair was the coiled cochlea as seen in man. It has been suggested that the coiling of the cochlea was likely to be a mechanism for accommodating a long papilla. This is in line with the fact that lizards mostly exhibit a cochlea length of not more than 2mm. All these alterations of the hearing mechanism and structures of the inner ear are however primarily a direct consequence of changes in the middle ear bone. They are largely a compensatory mechanism to accommodate the ossicle differences! While this diversity in ear structure clearly reflects hearing performance and abilities, it also raise the question of the advantages or disadvantages for animals to have sensitivity to a narrow range of frequencies(as seen in lizards) as opposed to having a wider range(man). Selection pressure on natural selection normally eliminates disadvantageous structural adaptations so what could be the reson for the persistence of the narrow range observed in lizards. A likely explanation is offered on the ground of trade-offs in nature. This has been aptly described by Coleman as 'A species that relies on high-frequency reception for survival might find it advantageous to avoid reception of low-frequency sounds that could mask more important high-frequency sounds.' (Coleman 2004 pp 1123). However most explanations for such functional consequences and characteristics till date are essentially speculations and would need further work to validate the speculations. 1.4 The Hearing apparatus as a topic of research (Reasons for choice of topic) "HE who wishes to decide whether man is the modified descendant of some pre-existing form, would probably first enquire whether man varies, however slightly, in bodily structure and in mental faculties" - Darwin This statement justifies the study of both the study of evolution and phylogeny by any student of biology. It is also an accurate summation of the need to study the evolutionary pattern of descent of man. Having said that, it becomes important to look at the main feature which differ amonst the various living species today. The auditory apparatus with it remarkable diversity across every phylum, class, sub class and species in the taxonomic set up is a perfect model to investigate with respect to evolution. A second important reason for studying the hearing apparatus is the relative difference of opinions in the scientific community with regards to its evolution. A classic example is provided by in the work of Gish. He has been known to claim that the absence of transitional forms between reptiles and mammals (either fossilized or living) is a major impediment to the present hypothesis that the mammalian middle ear bones are derived from the jaw bones in reptiles. (Gish 1978, pp 80). It is interesting to attempt to reach ones own conlusion regarding such discrepancies. This essay undoubtedly supports the alternative view that mammalian ear was indeed derived from the reptilian ear by the translocation of the middle ear bones and the references used throughout the body of the text evidence the same. The argument that some fossil form for the intermediate form is essential would need a complete fossil documentation for all the epochs which is yet to be achieved. Nonetheless discoveries like the complete skull of Hadrocodium wui (Luo et al. 2001 pp 1535) and cranial and jaw material from Repenomamus and Gobiconodon (Wang et al. 2001) are now building a complete picture of the evolution and to a large extent illuminate how and when the malleus, incus, and stapes completely detached from the lower jaw and evolved into the auditory ear ossicles. 1.5 Implications for further research/study and future directions The evolutionary pattern of the auditory apparatus as observed in reptiles and mammals has several far reaching implications the theory of common decent. It establishes in no uncertain terms the fact that descent of modern man (homo sapien) other primates, aves and reptiles is from a common ancestoral form. The evolutionary course of modification of the audiory system over the epochs is clearly indicative of the same. The pattern is also significant in terms of the concept of macroevolution whish suggests that large changes within biologic organisms are responsible for creation of a new phylum, it could also be termed as descent with modification and is in line with the theory of common descent. However as mentioned previously the fossil evidences (skulls, jaws or even ear bones) which would be a additional proof for the evolution are still incomplete and do not present a sequential picture, there are large gaps which are missing in the chain. It would be of much biological significance to obtain fossil record for the same. Another potential area of research would be to establish the cause for the formation of the tympanic middle ear in all lines of evolution during the Triassic period. While it is easy to understand the advantages of the separation of the chewing apparatus (jaws) from the ears, in the course of evolution from reptiles to mammals, the reasons for development and persistence of the middle ear tympanum remains elusive. Further paleontological studies could be directed to understand the selective advantage of this alteration of the auditory apparatus. Works Cited BBC, Journey of Life www.bbcworldwidetv.com Brown D. Evolution of Ear Is Noted in Fossil Transitional Stage of Organ May Have Helped Ancient Fish Breathe Washington Post Thursday, January 19, 2006 Coleman MN, Ross CF.Primate auditory diversity and its influence on hearing performance. Anat Rec A Discov Mol Cell Evol Biol. 2004 Nov;281(1):1123-37. Darwin, The descent of man and selection in relation to sex. 2nd edn., London, John Murray, 1882. Gilbert, S. F. (1997) Developmental Biology. Fifth edition. Sinauer Associates. Gish, D. T. (1978) Evolution The Fossils Say No! Public School Edition, San Diego: Creation-Life Publishers. Luo, Z.-X., A. W. Crompton and A-L. Sun. 2001. A new mammaliaform from the Early Jurassic of China and evolution of mammalian characteristics. Science 292: 1535-1540 Manley GA. 1990. Peripheral hearing mechanisms in reptiles and birds. Berlin: Springer Verlag. McGraw-Hill 2002 , Kardong 2002, pp. 274 Miller, M. R. (1992) in The Evolutionary Biology of Hearing, eds. Webster, D. B., Fay, R. R. & Popper, A. N. (Springer, New York), pp. 463-487. O'Neill, M. P. & Bearden, A. (1995) Hear. Res. 84, 125-138 Sellick, P. M. , Patuzzi, R. & Johnstone, B. M. (1982) J. Acoust. Soc. Am. 72, 131-141 Peck , J.E. Development of hearing. Part I: Phylogeny. J Am Acad Audiol. 1994 Sep;5(5):291-9. Wang, Y-Q, Y-M Hu, J Meng & C-K Li (2001), An ossified Meckel's Cartilage in two Cretaceous mammals and origin of the mammalian middle ear. Science 294: 357-361. Wartzok, D. and Ketten, D.R. 1999. Marine Mammal Sensory Systems. Pages 117-175 in Reynolds, J.E. III and Rommel, S.A. (eds.). Biology of Marine Mammals. Smithsonian Institution Press, Washington D.C. Werner YL, Wever EG. 1972. The function of the middle ear in lizards: Gekko gecko and Eublepharis macularius (Gekkonidae). J Exp Zool 179: 1-16. Werner YL, Rothenstein D, Sivan N. 1991. Directional asymmetry in reptiles (Sauria: Gekkonidae: Ptyodactylus) and its possible evolutionary role, with implications for biometrical methodology. J Zool Lond 225: 647-658. Yost, W.A. 1994. Fundamentals of Hearing: An Introduction. 3rd ed. Academic Press, New York, NY Read More