On page 158, Shubin begins discussion on ears. Explain why "some of the most distinctive traits of mammals lie inside the ear"(159). What is similar and different between mammalian inner ears and the inside of ears of other animals. How does the theme of evolution tie into the differences we see?
Matt Kim
(matthewkim0803@gmail.com)
It is said that "some of the most distinctive traits of mammals lie inside the ear" (159). This is because the mammalian middle ear contains bones that are different from all other animals. Mammals have 3 bones in the middle ear while reptiles and amphibians only have one. A lot of research and studies were done on this to figure out where the extra bones came from in the mammals. In 1837, Karl Reichert discovered that "the same gill arch that formed part of the jaw of a reptile formed ear bones in mammals" (160). This showed a new relationship between mammals and reptiles that seemed very hard to believe at the time: that the parts of ears of mammals were the same as parts of the jaw of reptiles. Then, in 1910 and 1912, Ernst Gaupp continued on this research. He discovered that the malleus and the incus "evolved from bones set in the back of the reptilian jaw" (161). This information was proven by W.K. Gregory who looked at African fossils and saw that "the bones at the back of the reptilian jaw got smaller and smaller, until they...lay in the middle ear of mammals" (162). All of this research that was done helped to prove the idea that the middle ear in mammals evolved from the jaw bones of reptiles.
ReplyDeleteIn mammals, the inner ear has different parts used for three main functions. One part is used for hearing, one for telling position, and one for telling the acceleration of the head. The inner ear is filled with a gel where specialized cells send hairlike projections. When the gel moves, the hairs bend and the nerve cells send an electrical impulse to the brain. Reptiles have a neuromast which is like the inner ear in mammals. As in the inner ear, the hair-like projections in the neuromast bend causing it to send signals back to the brain. However, it is uncertain which came first, the inner ear or the neuromast. More research needs to be done on this topic to figure out if the “inner ears arose from neuromast organs or the other way around” (170).
Sources:
Your Inner Fish
http://www.dailygalaxy.com/my_weblog/2009/09/your-inner-reptile-dna-research-links-human-ear-to-prehistoric-reptile-jaw.html
Danielle Webb (dwebb456@gmail.com)
Like Danielle mentioned, mammal ears are very unique in their bone structure. In fact, "the bones of the mammalian middle ear are like those of no other animal" (Shubin 160). What makes them different is that mammals have three bones whereas reptiles and amphibians have only one. Fish have none at all. The three bones in mammals are the malleus, incus, and stapes. German anatomist Karl Reichart discovered that the same gill arch that formed part of the jaw of a reptile formed ear bones in mammals. Darwin's theory of evolution put the pieces together and gave meaning to calling structures in two different species "the same," simply because the two species evolved from a common ancestor and thus, could have very similar structures. German anatomist Ernst Gaupp found that the single bone in the reptilian middle ear is the same as the stapes of mammals; both come from the second arch. Also, he found that the malleus and the incus evolved from bones set in the back of the reptilian jaw. W.K. Gregory found an important link between Gaupp's embryos and African fossils that were discovered: "the most reptilian of the mammal-like reptiles had only a single bone in its middle ear...it had a jaw composed of many bones" (Shubin 161). This showed that, over time, the bones at the back of the reptilian jaw got smaller and smaller, until they became the bones of the middle ear in mammals. This evolution of the ear from reptiles to mammals occurred because a three-boned ear allows mammals to hear higher-frequency sounds than animals with a single middle ear bone. This selective advantage perhaps helped mammals locate prey or predators, or communicate better within their own species with a new way of hearing. Thus, in developing this selective advantage, mammals would be able to survive and reproduce more effectively in their own environments.
ReplyDeleteOur inner ear has different functions: "one part is used in hearing, another in telling us which way our head is tilted, and still another in recording how fast our head is accelerating or stopping" (Shubin 164). The bony tubes inside the inner ear "are called the bony labyrinth filled with perilymph fluid, which the membranous labyrinth tubes are filed with endolymph. This is where the cells responsible for hearing are located" (http://www.medicinenet.com/script/main/art.asp?articlekey=21685). Like Danielle mentioned, our inner ear, filled with a gel where specialized cells send hairlike projections, is like the neuromasts of reptiles, where the hair-like projections bend and thus sned signals back to the brain. In addition, the part of our inner ear devoted to hearing is huge and coiled, while amphibians and reptiles have a simple uncoiled inner ear, which again shows that mammals obtained a new and better type of hearing because of environmental pressure and natural selection.
http://www.medicinenet.com/script/main/art.asp?articlekey=21685
Hannah Kay (hgkay@aol.com)
Given that all the questions, besides the evolutionary history, have been discussed in great detail, I will describe the history of the ear beginning in Tetrapods.
ReplyDeleteThe earliest tetrapods, amphibians and amniotes did not have ear drums. In fact ear drums evolved independently 3-6 times. In all the 3 major clades of amniotes (synapsids, eureptiles, and parareptiles) the stapes bones are relatively massive props that support the braincase, and this function prevents them from being used as part of the hearing system. As jaw bones evolved into Twin-jointed jaws during the early Triassic period, jaw bones were reduced giving rise to the intermediate between higher therapsid hearing and the hearing we see in mammals today. As the dentary continued to enlarge during the Triassic period, the older quadrate articular joint fell out of use. The resulted in the frequency range and sensitivity of the ear to be dependent upon the shape and arrangement of the middle-ear bones. In early synapsids the quadrate and articular had to function as the jaw joint, and this greatly limited how far these bones could be modified to alter the frequency range of the ear. However, once these bones were no longer involved in the jaw joint, variations which affected hearing would not also affect jaw joint function, and this allowed unconstrained evolution of the mammalian hearing apparatus. The following evolutionary pathways are some of the most well documented and lead to the conclusion that animals developed better hearing because enhanced hearing in the higher frequencies was helpful for nocturnal animals, in particular for detecting insects. This is the main reason for natural selection being a factor in the development of the ear.
In conclusion, we saw the structure of the jaw change which allowed for new structures to arise with new functions, in this case a well developed ear.
Eryk Fundakowski- arthur2446@comcast.net
http://www.johnholland.ws/home/humanearevolution
http://www.ncbi.nlm.nih.gov/pubmed/9310196
http://beta.revealedsingularity.net/article.php?art=mammal_ear