Monday, March 28, 2011
The Evolution of Smelling and Seeing
Chapters 8 and 9 discuss the structure, function, and evolution of the olfactory system and vision. In humans, many different genes are employed to detect odors where each gene forms a separate receptor. On the other hand, the human eye uses multiple genes that work together to form one organ that recognizes different images via several receptors. Both use numerous genes that together form one sensory organ. Compare and contrast the evolution of these two sensory organs. How do their differences in evolution reflect their differences in structure and function? Sami Kopinsky sami_kopinsky@yahoo.com
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ReplyDeleteAs evolution occurred from earlier mammals to humans, smell and vision have greatly changed. As mammals developed opsin proteins to see in full color, the size of their olfactory systems decreased (Pickrell). Therefore, advanced vision is a tradeoff for an advanced sense of smell. Most mammals that are not primates only are dichromats, which means they only have two different visual pigments. However, as mammals evolved, they became trichhomats, which have three kinds of visual pigments (Jacobs). This is an evolutionary advantage because having more pigments allows for better vision and the ability to see more colors. This could be particularly important for primates because they eat fruits, which are a variety of different colors. On the other hand, for animals that do not need to be able to discern similar colors from each other, a sharper sense of sell might actually be more advantageous, especially for animals that use their sense of smell to capture prey.
ReplyDeleteThe most important part of the structure of the olfactory system is that epithelial create a continuous supply of new neurons. Receptor cells in the olfactory system have a dendritic rod and cilia, which provides a large surface area (Vokshoor). This great surface area is great structure for the olfactory system so that it can sense stimuli more easily. The most important parts of the eye structure that help with vision are the retina, iris, and cornea. These structures allow only a certain amount light to enter the eye and help to the eye to focus on the image. Proteins are also involved in more complex vision. In terms of evolution, the structures of the eye and olfactory system are very different because the two systems have evolved to serve different functions. However, both of their structures are more advantageous for each system to be able to form its specific task.
Sources:
http://genome.wellcome.ac.uk/doc_wtd020881.html
http://www.scientificamerican.com/article.cfm?id=evolution-of-primate-color-vision
http://emedicine.medscape.com/article/835585-overview
http://www.myeyeworld.com/files/eye_structure.htm
Marissa Lobl
marissa.lobl@gmail.com
As Marissa said, smell and vision have greatly changed over the last millions of years. But I would like to elaborate on how much more complicated these senses have become over time. First off, the ability of eyesight. Sight begins as light waves from an object enter the eye first through the cornea, which is the clear dome at the front of the eye. The light then progresses through the pupil or the circular opening in the center of the colored iris. Fluctuations in incoming light change the size of the eye’s pupil. This is why when light entering the eye is bright, the pupil will constrict. This is called the papillary light response. Initially, the light waves are bent or converged first by the cornea, and then further by the crystalline lens (located immediately behind the iris and the pupil), to a nodal point (N) located immediately behind the back surface of the lens. At that point, the image becomes reversed (turned backwards) and inverted (turned upside-down). The light continues through the vitreous humor, the clear gel that makes up about 80% of the eye’s volume, and then, ideally, back to a clear focus on the retina, behind the vitreous. The small central area of the retina is the macula, which provides the best vision of any location in the retina. If the eye is considered to be a type of camera, the retina is equivalent to the film inside of the camera, registering the tiny photons of light interacting with it. Within the layers of the retina, light impulses are changed into electrical signals. Then they are sent through the optic nerve, along the visual pathway (neurons (dendrites, axons) etc.), to the occipital cortex at the posterior (back) of the brain. Here, the electrical signals are interpreted or “seen” by the brain as a visual image.
ReplyDeletehttp://www.livescience.com/3919-human-eye-works.html
Next is the process of smell. The olfactory system is activated when molecules come in touch with specialized olfactory vesicles. The molecules are then carried to the epithelium where more than 100 million olfactory receptor cells reside. The epithelium consists of three cell types: basal, supporting, and receptor. Basal cells are simply stem cells that will eventually give rise to olfactory receptor cells. The supporting cells are scattered among the receptor cells and help empty contents on to the mucosal surface. Receptor cells are actually bipolar neurons with each possessing their dedrtitc rod that helps provide the transduction surface for stimuli. The neurons have small unmyelinated axons. Because of this, conduction velocities are very slow and support is provided by a Schwann cell. These axons then serve as the pathways for electrical messages to be sent from neuron to neuron inside the brain.
http://emedicine.medscape.com/article/835585-overview#aw2aab6b6
Matt Micucci (coochqbk@sbcglobal.net)
The olfactory system and vision are both, clearly, extremely important senses to humans. Marissa and Matt did a sufficient job in explaining how the eyes and our sense of smell works, but I would like to, instead, contrast the importance and evolution of both these senses in greater detail. The eye, something that has been changing for millions of years, is perhaps the most complex of the senses in the human body, in fact, even Shubin explained how we now rely on vision more than we do smell (Shubin 147). At first, eyes were placed on the sides of the head of fish so the fish would have a 360 degree view all around them, but now that we have a neck, this is unnecessary and now eyes are placed at the front of face. When looking at the progression of eyes and sight, human eyes are arguably the most complex of them all. The human’s sense of smell, on the other hand, is certainly not the most complex. Many organisms make up for their lack of strong vision in the field of the olfactory senses. A dog, for example, has a strong smelling abilities, while they can’t see in color as we can.
ReplyDeleteOur color vision comes from the fact that we have cones. Like Marissa had mentioned, visual pigments called photopsins allow us to see in color (Campbell 1102). What I found misleading, however, is when Marissa talked about primates needing color vision because they eat fruits. Yes, this is true, but this isn’t why they need to see in color. The shift in receptors to allow us to see color may have changed when the flora in our earth changed. Color vision was a selective advantage for monkeys because when they lived on trees, the color vision “enabled them to discriminate better among many kinds of fruits and leaves and select the most nutritious among them” (Shubin 153). The different coloring of food symbolizes different nutrition values, and obviously certain animals benefit from certain foods, therefore the monkeys with color vision had the advantage. So in response to Marissa, yes the monkeys need color vision to eat fruits, but they got this not so they could differentiate between what was a fruit or what wasn’t, but rather, what has a higher nutrition level. It wasn’t necessarily because of a shift in the ape’s diet so much as it was a shift in the coloring of the flora surrounding them.
What both Marissa and Matt spoke about, but never established the connection, is the use of receptors in both sight and smell. “Each of the 5 senses consists of organs with specialized cells that have receptors for specific stimuli. These cells have links to the nervous system and thus to the brain” (http://www.scientificpsychic.com/workbook/chapter2.htm). As the evolution of both smell and sight progressed, all the different receptors had to connect in some way all together -- that being our brain. So although the evolution of sight and smell happened independently, they had to work with our bodies and take use of our receptors so it could be reconnected to the brain -- the organ that connects it all.
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-Michelle Layvant, michellel94@hotmail.com