In chapter 4, Shubin talks about the development of teeth and hard skeletons in animals. Shubin states that "for years, paleontologists have argued about why hard skeletons...arose in the first place" (76). He says that scientists that believe that skeletons began with "jaws, backbones, or body armor" believe that hard bones (like teeth) arose for the purpose of eating other animals. What are some other reasons that paleontologists might argue for why hard skeletons arose? Would these paleontologists also believe that skeletons began with jaws, backbones, or body armor? If not, what do you think they believed the skeleton began with?
Danielle Webb
(dwebb456@gmail.com)
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ReplyDeletePaleontologists may argue that hard skeletons arose to assist the movement of organisms from the water to the land. These paleontologists would probably argue that hard skeletons began with body armor, such as exoskeletons. Two key features of hard skeletons make it likely that exoskeletons arose for this reason include prevention of desiccation, and assisting in locomotion. An interesting case to look at is arthropods. Arthropods have a hard exoskeleton as part of their body plan. The arthropod exoskeleton has helped allow the arthropod to exist on the land because it is relatively impermeable to water and prevents desiccation (Campbell 685). By looking at arthropods, the most diverse group of invertebrates, scientists agree that having a hard skeleton allows for easier movement and diversity (Terkanian, B). The way that the exoskeleton helps the animal move is by providing points of attachment for muscles that move appendages (Campbell 685). The combination of being able to exist in a dry environment and having advanced locomotion emphasize the importance of the evolution of hard skeletons.
ReplyDeleteSince paleontologists have argued that hard skeletons arose for the purpose of eating other animals, it seems likely that they could also argue that hard skeletons arose as protection for animals from being eaten by predators. These paleontologists would probably believe that skeletons began with body armor because body armor is good protection against being eaten. Although Schubin discusses that hard skeletons did not at first arise to help prey avoid predators, it is possible that this helped advance and develop hard skeletons.
While thinking about the evolution from invertebrates to vertebrates, it is clear that the main difference is that vertebrates have a hard backbone. It is also important to realize that this has caused vertebrates to become much larger and advanced. This shows that evolution has favored animals with hard skeleton, such as a back bone. Also, in terms of surviving predators, it is clear that evolution has favored animals with hard teeth and jaws (to capture prey to survive) and animals with armor (to survive attacks by predators). A reason that freedom of motion provides an evolutionary advantage for animals with hard skeletons is because it could help them move more quickly to escape predators or natural disasters, or to more efficiently hunt for prey (Terkanian, B).
Sources:
http://www.desertmuseum.org/books/nhsd_look.php?print=y
AP Biology- Campbell/Reece
Marissa Lobl marissa.lobl@gmail.com
Marissa made excellent points by describing how hard skeleton provides protection and freedom of movement, however, I think she could have gone a little more specific to tell why hard skeleton helps in movement. It is known that fish that live in water contain bony endoskeletons, but certain fish like sharks contain an endoskeleton made of cartilage, and this is a huge benefit for sharks. One huge benefit for having a cartilage endoskeleton is because of the light weight. Cartilage is half as dense as water yet it still has the property to be tough and supportive (1). When in water, fish must constantly swim or suck in gases to stay afloat so their weight must always be watched and managed so that they do not sink. However, on land the animals can have the hard skeleton because they are standing on a firm ground and weight would not matter. This brings in the theme of structure and function. Cartilage in sharks is made of collagen which provides the flexibility and lightweight attributes to help sharks movement and swim for predation. Hard bone is made of calcium and phosphorus which makes it hard and barely flexible allowing for sturdy support and the cost of heavy weight (2). For that reason, it may very well be possible that that is the main reason why land animals developed the hard skeleton.
ReplyDeleteThe theme of evolution is also very present in this topic. The reason why having cartilage is a selective advantage for water animals, like a shark, is because of how they move and the energy needed to move. Because cartilage is so light, energy costs are low allowing the shark to go without a meal for a while because sometimes in the ocean food source may be scarce. But because the food is scarce, having a cartilage internal skeleton is beneficial for fast swimmers like sharks. The movement of the tail left and right to propel the shark forward, guided by all the fins, requires flexible bones like cartilage. The flexibility of the cartilage allows the shark to rapidly move their tail in times when speed is needed giving the shark an advantage in predation (3). However, as Shubin suggests throughout this book, we all have our inner fish, and through evolution we can clearly see that the use of a soft flexible skeleton would have no benefits living on firm hard land. Therefore, evolution took its course and developed the hard bone as the support for the immense weight that land animals can have. The purpose of hard bone allows animals to push off from their feet and support themselves upright without being flimsy like jelly. Hard bone is a selective advantage for land animals because it enables to run and push off with force for predation and as a means of escape. To bring in physics, we, as humans, are able to put more force on our feet and toes and propel us forward continuously instead of having cartilage that can easily break because of its flexibility. You can easily see in pictures of human ancestry how evolution on hard bone as taken its course (4). At first humans were on all fours and short, but as time went on, humans grew taller and were able to stand upright all because of the development of hard bone.
1. http://www.elasmo-research.org/education/topics/p_cartilage.htm
2. http://www.astrobio.net/exclusive/3733/skeletons-in-the-pre-cambrian-closet
3. http://animals.howstuffworks.com/fish/shark2.htm
4. http://www.impactlab.net/2007/12/11/human-evolution-speeding-up/
Benny Jeong
bennyjeong218@gmail.com