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The evolution of mammalian auditory ossicles is one of the most well-documented and important evolutionary events, demonstrating both numerous transitional forms as well as an excellent example of exaptation, the re-purposing of existing structures during evolution. In reptiles, the eardrum is connected to the inner ear via a single bone, the columella, while the upper and lower jaws contain several bones not found in mammals. Over the course of the evolution of mammals, one lower and one upper jaw bone (the articular and quadrate) lost their purpose in the jaw joint and were put to new use in the middle ear, connecting to the stapes and forming a chain of three bones (collectively called the ossicles) which transmit sounds more efficiently and allow more acute hearing. In mammals, these three bones are known as the malleus, incus, and stapes (hammer, anvil, and stirrup respectively). The evidence that the malleus and incus are homologous to the reptilian articular and quadrate was originally embryological, and since this discovery an abundance of transitional fossils has both supported the conclusion and given a detailed history of the transition. The evolution of the stapes was an earlier and distinct event.〔(The Shoulder Bone’s Connected to the Ear Bone… ) Carl Zimmer blog ''The Loom'' at ''Discover'' magazine for 2008 October 15.〕 ==Reichert–Gaupp theory== Following on the ideas of Étienne Geoffroy Saint-Hilaire, the relationship between the reptilian jaw bones and mammalian middle-ear bones was first established on the basis of embryology and comparative anatomy by Reichert (in 1837, before the publication of On the Origin of Species in 1859) and advanced by Gaupp, and this is known as the ''Reichert–Gaupp theory''.〔 〕 In the course of the development of the embryo, the incus and malleus arise from the same first pharyngeal arch as the mandible and maxilla, and are served by mandibular and maxillary division of the trigeminal nerve. ...the discovery that the mammalian malleus and incus were actually homologues of visceral elements of the "reptilian" jaw articulation ... ranks as one of the milestones in the history of comparative biology.〔 Novacek references these early works: – – 〕 ... it is one of the triumphs of the long series of researches on the extinct Theromorph reptiles, begun by Owen (1845), and continued by Seeley, Broom, and Watson, to have revealed the intermediate steps by which the change may have occurred from an inner quadrate to an outer squamosal articulation ... Yet the transition between the "reptilian" jaw and the "mammalian" middle ear was not bridged in the ''fossil'' record until the 1950s with the elaboration of such fossils as the now-famous ''Morganucodon''. There are also more recent studies in the genetic basis for the development of the ossicles from the embryonic arch, and relating this to evolutionary history. "''Bapx1'', also known as ''Nkx3.2'', is the vertebrate homologue of the ''Drosophila'' gene ''Bagpipe''. A member of the NK2 class of homeobox genes ...", this gene is implicated in the change from the jaw bones of non-mammals to the ossicles of mammals.〔A survey of the genes involved in the development of the vertebrate middle ear is given in 〕 Others are ''Dlx'' genes, ''Prx'' genes, and ''Wnt'' genes. Image:Mammal middle ear.png | thumb | right | 251px | A typical mammalian middle ear: sound makes the tympanum (eardrum) vibrate; 3 small bones, the malleus, incus and stapes, transmit the vibrations to the labyrinth (inner ear), which transforms the vibrations into nerve signals. # rect 30 14 118 28 Tensor tympani rect 146 48 181 60 Incus rect 169 65 227 77 Stapedius rect 177 100 231 113 Labyrinth rect 164 116 205 130 Stapes rect 154 136 242 151 Tympanic cavity rect 165 165 226 194 Eustachian Tube rect 18 164 132 194 Ear drum rect 5 138 87 151 Ear canal rect 28 56 74 71 Malleus # desc top-right 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Evolution of mammalian auditory ossicles」の詳細全文を読む スポンサード リンク
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