Neil Shubin indicates that a human's sense of smell can capture the scent of a "bell pepper at a concentration of less than one part per trillion". (141) How did the evolution of the acute sense of smell aid individuals in survival? Fish also have a sense of smell which they use to locate and capture food. Explain how the sense of smell evolved to accomodate life on land and how it compares to the sense of smell that a fish has. Is it possible that what we label odorless really has an odor that we can't yet detect? Why is the sense of smell of humans less refined than that of a dog?
Yekaterina Khavkhalyuk (kittykatx93x@yahoo.com)
Smells help individuals survive. Shubin explained how smell of tasty food can make us hungry, while the smell of something like a sewage can make us feel ill -- thus causing us to avoid/be attracted to certain things. In fact, "our sense of smell allows us to discriminate among five thousand to ten thousand odors" (Shubin 141). Humans have an extremely acute sense of smell because it is a lock-and-key mechanism. The odor molecule serves as the lock and the receptor on the nerve cell serves as the key. The acute sense is caused by each receptor tuned to a different kind a molecule (this same idea can be seen throughout the body, no solely smell, but also, as we just recently talked about: B cells!)
ReplyDeleteThe discovery of the evolution of smell was facilitated by Buck and Axel's experiment. They found that 3% of a human's genome is devoted to genes for detecting different odors. And following this discovery many began to search and find other species with the same receptor genes.There are two types of receptors: water-based and air-based. Our sense of smell transitioned from being water-based like that in fish to air-based. This could have happened because of the increase in oxygen in the environment. Millions of years ago, there was very little oxygen in the atmosphere, but this eventually changed. These environmental pressures provided the perfect opportunity for the fish living in the "fish-eat-fish" world to start moving out of the water. This of course didn't happen within one generation, but after many years the transition was a success. Part of the transition was readjusting the sense of smell. The water-based receptors in the nasal neurons had to shift into air-based ones.
Shubin explained how jawless fish have a small number of odor genes, but bony fish have more, and then amphibians and reptiles have even more, and then finally coming to us where we have very many (Shubin 145). This high number makes us specialized smelling animals. The large amount of odor genes of a mammal arose by many rounds of duplication of the small number of genes present in primitive species. And as time goes on, many of them even become functionless. Because our smell came from the evolution from water-based receptors, many of the specialized genes are unnecessary when it comes to air-based receptors.
Although our sense of smell is developed compared to many other organisms, a dog has a sense of smell more refined than ours. This is because we have traded smell for sight. Our eyes are more developed than our sense of smell and we rely on it more, while dogs don't have very developed sight. "The percentage of the dog's brain that is devoted to analyzing smells is actually 40 times larger than that of a human! It's been estimated that dogs can identify smells somewhere between 1,000 to 10,000 times better than...humans can" (http://www.dummies.com/how-to/content/understanding-a-dogs-sense-of-smell.html). Essentially, it is a tradeoff for humans to have a sense of smell less developed but a sense of sight more developed.
And finally, in response to the question regarding whether or not it is possible that we label odorless really something that has an odor we can't yet detect and this question is similar to one that is asked if a tree falls in a forest with no one around does it make a sound. The answer is: no it doesn't have an odor to us because we don't have the certain receptor for it, but if another organism were around, like a dog, and it had a receptor for that scent, then yes, to that dog it has an odor.
-Michelle Layvant, michellel94@hotmail.com
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ReplyDeleteSmell, or the ability of neuron receptors to send a signal to the brain based on the molecule it receives is an important ability to organisms whether they are predator or prey. Animals like the fish and humans have a very acute sence of smell that can smell over time. And though it can dected a "bell pepper at a concentration of less than one part per trillion", a human’s sense of smell isn’t as accurate as many other animals (141). Different organisms have developed different senses of smell over time. For example, mammals can have scent glands in different locations, such as male deer that have them on the lower legs or elephants that have them behind their eyes (http://www.earthlife.net/mammals/smelly.html). The main way that reptiles can smell is using Jacobson’s organ. This organ is a chemoreception organ found in amphibians, reptiles and some mammals and is vestigial in humans. Snakes and other reptiles flick substances into Jacobson’s organ using their tongue(http://chemistry.about.com/cs/medical/a/aa051601a.htm). This is one difference between an animal that is more water based then air based.
ReplyDeleteThe fish’s sence of smell is even more water based. They have holes, which look like nostrils, but are called Nares. Fish do not breathe through these holes and they don’t lead to the throat like in mammals, they do lead to sensory organs that pick up the chemicals and send the signal to the brain (http://www.seaworld.org/aquademics/tetra/all_about_fish2.htm). Since the nose in a fish is only used for smelling, it is very good at it as seen with sharks who can pick up one part fish scents per billion parts (http://animals.howstuffworks.com/fish/shark-senses1.htm). This development of smelling over time would be more beneficial in a fish than in a mammal, perhaps showing the divergence in evolutionary history.
For this reason, animals have developed different abilities of smelling, for the many reasons that they use it for. Being able to smell sex pheromones ( Cambell 1090) or odorants in food(Cambpell 1097) is a great advantage to have when attempting to survive and reproduce.
Alex Sapozhnikov (marijio@gmail.com)