In chapter 4, Shubin explains the different sort of teeth starting from the incisors, continuing with the molars, etc. He also goes on to say that the mineral hydroxyapatite is what gives teeth its “hardness.” Finally, Shubin ends by saying on page 79, “We would never have scales, feathers, or breasts if we didn’t have teeth in the first place.” Clearly, teeth are one of the most important features of an animal’s anatomy, but explain the transition from teeth within a fish to teeth within a terrestrial animal? What role does hydroxypatite play in teeth as you move up the food chain (such as the example Shubin provided with clams and lobsters having calcium carbonate and chitin instead)? Finally, why exactly would we never have scales, feathers, or breasts if teeth didn’t exist?
-Michelle Layvant, mlayvan2@students.d125.org
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ReplyDeleteTeeth are indicative of an organism's diet. As human ancestor species moved from the water to the land, their diet also changed. To begin with fish, it is clear that they have a vastly different diet then that of a terrestrial animal. The size and position of the mouth of a fish have developed to maximize food-gathering potential, and mirror a fish’s eating habits and preferred diet (http://scienceray.com/biology/zoology/understanding-mouth-structure-of-fish/). So even in the grouping of fish there is a significant amount of variety as far as tooth structure. For example, in bony fish, teeth consist of an enamel coating over dentine, surrounding a pulp cavity filled with nerves, blood vessels, and connective tissue. In carnivorous fish, teeth are sharper, just as in carnivorous terrestrial animals. Herbivores that live on land tend to have large, mostly flat round teeth. They lack the incisors and canines of the carnivorous creatures (http://www.blm.gov/id/st/en/prog/wildlife/herbivores.html). This idea ties to the theme of evolution. Over time, the carnivorous terrestrial population developed sharper teeth so that those species of animals could engage in mechanical digestion of their food. As the genetic code was altered, those individuals with the sharper teeth were more able to obtain nutrition, and thus survive longer and reproduce, whereas carnivorous terrestrial individuals without sharp teeth would have faced difficulty in trying to maintain their diets. As the individuals without sharp teeth were eliminated, the population as a whole would come to possess sharp teeth. Moreover dental and jaw structure as a whole relates to the idea that structure determines function, because sharp incisors and canines allow for mechanical digestion of other animals.
ReplyDeleteShubin discusses the reasons that hydroxyapatite arose in the first place. The presence of hydroxyapatite first arose in teeth, not just for protection, but for the consumption of other animals (Shubin 76). Terrestrial animals developed hydroxyapatite in their teeth because of their changing diet. This difference in lifestyle can explain the disparity between human teeth and the calcium carbonate or chitin of other organisms, which have vastly different diets. Hydroxyapatite allows animals to mechanically digest the tough, resistant defenses of the animals that they eat.
Teeth develop by an interaction of two layers of tissue in our developing skin (Shubin 78). Shubin draws the connection between the process of tooth development and the development of scales, hair, feathers and various glands. These processes occur with similarity. Shubin notes that two layers of the tissue in developing skin fold together and secrete proteins (Shubin 78). He also notes that the same genetic switches are active in the development of teeth, hair, scales, feathers, and glands. The point that these other characteristics wouldn’t have developed in the absence of teeth is validated by the idea that the same genes code for the development of these characteristics. If the genes that code for the development for teeth were absent, and those same genes code for the development of hair and glands, then the organism would be unable to develop specific characteristics.
Vickram Pradhan 1/2A
vickram.pradhan@yahoo.com
I'm posting two responses in a row because as one big post it surpassed the character limit.
ReplyDeleteAs Vickram alluded to, hydroxyapatite developed in teeth far before it became a prominent element in bones. He went on to say that this development arose as the diet of animals changed. However, I believe that it is here that he misspoke. His first mistake was in saying that hydroxyapatite developed as teeth in terrestrial animals because of their changing appetites. Hydroxyapatite actually first developed as teeth in fish. Although a seemingly small and innocent mistake, it actually has a monumental impact on evolution. The first teeth provided a selective advantage to those that possessed them. Never before had it been easier to digest food. According to Darwin’s theory of natural selection, this advantage was passed down generation to generation since those with the advantage had a better ability to directly access resources, in this case food. As those with the advantage become more prominent, those without teeth began to die out due to competition for resources. Thus, fish with teeth became more abundant and their prey became easier to attain.
When a predator evolves to better hunt their prey, a reciprocal process called coevolution often takes place. Coevolution is when an evolutionary change in one organism leads to a reciprocal change in another organism. The types of relationships that produce these changes are most often either 1) predator vs. prey, 2) parasite vs. host, 3) competitive species or 4) mutualistic species (1). In the case of the development of teeth and bones, coevolution occurred due to a predator vs. prey relationship. As fish evolved to develop teeth, their prey needed a way to better protect themselves. This gave rise to organisms like Ostracoderms. They developed an exoskeleton to deter their predators. As can often happen, the best way to protect oneself against a new form of attack is to use the attacker’s new found gift against them. So, Ostracoderms evolved to develop an armor literally made up of tiny teeth. This also gave them a selective advantage, leading to natural selection and an increased amount of fish with protective exoskeletons. This type of relationship where a change in the predator or prey affects the other’s ability to survive and reproduce and leads to another change in the other party often leads to what is known as a coevolutionary arms race (2). Shubin alludes to this as well (pg 76). In response to the Conodonts developing teeth, the Ostracoderms developed armor. Then fish developed jaws to crack the armor, and their prey developed stronger armor. This arms race exponentially increases the effects of a single selective advantage. Furthermore, it quite possibly could have continued to lead to the development of land animals to avoid bigger, stronger predators in the water as well as the possibility that it led to terrestrial animals that needed to find weaker prey outside the water.
Another aspect of the evolutionary process that Vickram missed on is the precursor to teeth. Before Conodonts, the first ever vertebrates were Lampreys. As the oldest living vertebrates, they offer a unique view into feeding mechanisms before teeth. Most Lampreys are parasites. They have similar bodies to Conodonts and lack appendages. They feed by latching onto the side of a live fish. They then use their rasping tongues to break through their host’s skin and then proceed to ingest the blood of their victim (Campbell 704). The most important aspect of the Lamprey from an evolutionary aspect is most likely their tongue. That they swam the way they did and had a parasitic relationship with their prey is not surprising. However, that they used their tongue to penetrate their victim’s skin obviously appears as inferior to the damage that teeth could ensue. One interesting possibility that could have led to the development of teeth could be that the prey of the Lamprey developed thicker and harder to penetrate skin. This could have caused a coevolutionary response in the Lamprey to develop teeth. This would suggest that the coevolutionary arms race actually began well before teeth were factored in.
ReplyDelete1. http://evolution.berkeley.edu/evosite/evo101/IIIFCoevolution.shtml
2. http://biomed.brown.edu/Courses/BIO48/27.Coevolution.HTML
-Robbie Thomashow
(diehardcubsfan93@comcast.net)