Sujin Ko (sujinko93@gmail.com)
Monday, March 28, 2011
TEETH (humans vs. reptiles)
In the chapter titled "Teeth Everywhere," Shubin describes the detail and precision of human teeth, having "upper and lower cusps, basins, and ridges match closely." (61) Unlike humans, reptiles, such as crocodiles, snakes, and lizards, have teeth that are all very similar to each other; a blade-like shape with the only difference being the size of the teeth. Why is it that reptiles are able to successfully survive without precise upper and lower teeth that fit together when humans cannot? What are the selective advantages of reptiles having a basic, similar structure for all teeth? what are the selective advantages of humans having precisely matching teeth? Explain with specific, detailed examples.
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The teeth found in reptiles like crocodiles, snakes, and lizards are all shaped like the Canines found in human mouths. Canines are the teeth that are the sharpest of all the different types of teeth found in the human mouth: Incisors, Canines, Premolars, and Molars. Incisors are used to bite and cut food. Canines are used for ripping and tearing food apart. Premolars, also called bicuspids, are used for chewing and grinding food. Molars are used to grind and macerate food.
ReplyDeleteEach different tooth is shaped in a way that fits its function. For example, an herbivore’s mouth would only contain molars to chew leaves. Leaves would be virtually impossible to chew if an herbivore’s mouth contained canines or incisors. Teeth are a prime example of the biological theme of the Relationship between Structure and Function. Humans are omnivores, so a collection of all types of teeth are needed in order to be able to chew the meats and greens. Humans need to chew their food into a ball of food mush called a bolus, and a proper occlusion, alignment of the teeth and jaws, is necessary in order to do so. Reptiles, however, are able to swallow their food whole, so molars and premolars aren’t needed in order to swallow food. Instead of molars and premolars, the reptilian mouth only contains canines to help rip through flesh and hold food in place while it is being swallowed. All in all, reptiles are able to survive without precise upper and lower teeth alignment because they don’t need to chew their food.
An advantage to having teeth that are all similarly shaped is that they all do one task EXTREMELY well: slice through flesh and hold food in place. Having a mouth that only contains canines is a very effective trait for reptiles because they don’t need to swallow their food; they only need to keep it in place while it is being swallowed. Why would they need molars if they never need to chew? A very good example of the effectiveness of a mouth full of canines is contained in this video: (http://www.youtube.com/watch?v=PiG3yokCOug). The crocodiles use their extremely sharp teeth to rip through the wildebeests’ flesh and hold them in place until others come to help finish the kill. The wildebeests are far bigger than the crocodiles, but with a mouth full of canines, crocodiles are able to hold onto them and eventually kill them. Again, the relation between the shape of a canine and its job reflects the Relationship between Structure and Function. A sharp and thin structure makes canines ideal for their function: to rip through flesh and hold prey in place.
An advantage to having teeth that fit perfectly together is being able to eat a large variety of food without having to swallow it whole. Incomplete digestion occurs within humans if food isn’t chewed properly. Nutrients aren’t completely absorbed, and a condition called bacterial overgrowth may result from undigested food entering the colon. This condition often leads to nausea, indigestion, and diarrhea. Being able to properly chew food is very beneficial to humans, and has been ever since the human population has adopted such a proper occlusion. Before agriculture was performed, ancient humans were hunter-gatherers. Hunter-gatherer lifestyles usually consisted of cycles of feast and famine, and food had to be thoroughly absorbed in order to survive the cycles of famine. One major factor in surviving these cycles of famine could be the proper alignment of teeth. Ancient humans with poor teeth alignment wouldn’t have been able to survive the cycles of famine nearly as well as those who were able to properly chew food with perfect teeth alignment. The event of natural selection would eventually out breed the ancient humans with poor teeth alignment, and those with proper teeth alignment survived and reproduced. Proper teeth alignment has been a selected advantage for humans for thousands and thousands of years.
Mikey Ling
(mikeyling@ymail.com)