In chapter ten titled, "Ears", Shubin discusses how ears have evolved from being part of the jaw in reptiles to becoming a complex three bone ear in mammals. Shubin explains how the different bones in the reptiles jaw changed to become part of the ear as mammals evolved. Using the theme of evolution, explain why this may have happened. Does it have anything to do with the lifestyle of the animal? The feeding mechanisms? What does this change of function in jaw bones tell us about the connection between reptiles and mammals?
Marissa Lobl marissa.lobl@gmail.com
Shubin states on pg. 164 that "hearing in water is different from hearing on land, and the small size and position of the stapes [one of the three small bones of the inner ear] makes it ideal for picking up vibrations in the air. The new ability came about by modifying the upper jawbone of a fish." This idea of using old parts in new ways, as in the evolution of large rods in a reptiles’ jaws into small bones in a mammal’s ear, is an example of divergent evolution. This was a necessary process to create more biologically fit organisms because the different size and new placement of these bones allowed for “mammals to hear higher-frequency sounds than animals with a single middle ear bone…, new patterns of chewing,… [and] new ways of hearing” (Shubin 162).
ReplyDeleteOne reason that “fish and sharks do not have ears” and thus can not hear (Shubin 162) may be because they lack a cochlea. Pressure waves, which is the substance of our hearing must “pass around the apex (tip) of the cochlea… [to prevent] reverberating within the ear and causing prolonged sensation” (Campbell 1094). Seeing as how a fish skeleton “consists of cantilever ribs extending out from a backbone[,]… acoustic waves [would have] to be introduced to the diaphragm placed at one end of the backbone to travel in one way along the backbone” (Proc. SPIE Vol. 3514, p. 266-275, Micromachined Devices and Components IV, Patrick J. French; Kevin Chau; Eds, http://adsabs.harvard.edu/abs/ 1998SPIE.3514 ..266H) Needless to say, this would be an evolutionary disadvantage for fish seeing as there are pressure waves from the water in the ocean itself constantly. Without a cochlear structure, these reverberations would be relentless annoyance for fish. More than that, though, these reverberations would travel all they way down its backbone, instead of the compact structure of the inner ear that benefits mammals, which would probably inhibit swimming.
Therefore, as fish began to live on land, it became for advantageous and necessary for them to hear. A change from a terrestrial diet from an aquatic one also caused the evolution from a long-set jaw to a smaller one, and thus the two bones—malleus and incus—became available for use in the inner ear. The fossil record Shubin discusses on page 161 confirms this notion seeing as how the 1840s brought ample evidence of “a beautiful transition series… between reptile and mammal.”
(Jackie Edelson, jedelson92@gmail.com)
In the ears of mammals, they have the stapes bone, which are also found in reptiles. However, two extra bones – the malleus and the incus – are also found in mammals but not reptiles. These bones “evolved from bones set in the back of the reptilian jaw” (161). The reptilian jaw and the fish jaw was a series of modified gill arch bones that was needed for prey processing. As reptiles evolved into mammals, however, the need for this large jaw wasn’t as necessary and so the jaw bones could become smaller. This development in mammals helped mammals to hear higher frequencies than the reptiles, as Jackie stated. However, the development also helped improve the teeth and the formation of the mouth in mammals. Since the “bones of the jaw [got] smaller and move[d] to the ear” (62), it left more room for the teeth of mammals. This allowed for their teeth to be able to fit precisely together. This development in mammals allowed for them to “break up food with maximal efficiency” (61). This proved to be a selective advantage in animals as they started moving to land and soon, it was necessary for their teeth to have precise occlusion because “a mismatch between upper and lower teeth [could] shatter our teeth” (61).
ReplyDeleteThis change in function in jaw bones shows how reptiles and mammals are connected and how the mammals evolved from reptiles. These connections are seen in the African fossils founded by W.K. Gregory which showed “the bones at the back of the reptilian jaw [getting] smaller and smaller, until they ultimately lay in the middle ear of mammals” (162). So, the change in the jaw and ear structures from reptiles to mammals shows connections between these animals, but it also shows the differences between them as Jackie showed when explaining why fish and sharks do not have ears.
Sources:
http://darwin.bio.uci.edu/~edrucker/home/feeding.htm
Danielle Webb (dwebb456@gmail.com)
Everything that Jackie and Danielle “Bieber-lover” Webb said was completely right. I would just like to add on a little bit more detail as to how the process of smelling actually works. It begins when airborne molecules stimulate the olfactory epithelium which is roughly one square inch of surface area within your nose. Mucus, secreted by the olfactory gland, coats the epithelium’s surface and aids in the dissolving of odorants. The receptor cells within the epithelium are neurons with knob-shaped tips called dendrites. There are roughly 100 million receptor cells within the olfactory epithelium. The dendrites are covered with olfactory hairs that bind with odorants and then send an electrical impulse to the olfactory bulb through the axon at its base. The pulse then travels to the brain where it is interpreted as a sense through an innumerable amount of other neurons.
ReplyDeletehttp://health.howstuffworks.com/human-body/systems/nose-throat/smell2.htm
http://emedicine.medscape.com/article/835585-overview#aw2aab6b6
How the body interprets the smell was still very unclear until it was deciphered for the most part using molecular techniques. In 2004, Richard Axel and Linda Buck won the Nobel Peace Prize for their work in Physiology and Medicine because of their discovery of a large gene family (about 1,000 different genes) that produced roughly 1,000 olfactory receptor types. This shows that each olfactory cell has only one type of odorant receptor which explains why some people cannot smell certain smells. Because of this, our olfactory cells are highly specialized for a few odors. These olfactory cells send nerve impulses through micro domains, glomeruli, through the primary olfactory area, and into other parts of the brain where the impulses form a pattern which is then interpreted by those sections of the brain into our perception of a smell.
Axel and Buck also cloned olfactory receptors to show that they belonged to the family of G protein coupled receptors. These receptors are activated by light-sensitive compounds, odors, pheromones, hormones, and neurotransmitters, and can vary in size from small molecules to peptides to large proteins. They are also the target of approximately 30% of all modern medicinal drugs.
http://nobelprize.org/nobel_prizes/medicine/laureates/2004/press.html
Matt Micucci (coochqbk@sbcglobal.net)