Aparna Pal
aparna.pal.1994@gmail.com
Friday, April 1, 2011
Old parts in new ways
Throughout the book, Shubin has described different ways organisms have evolved by using pre-existing parts in a different way. Find at least three examples throughout the book and explain how they helped the organism gain an advantage. Describe any setbacks that were later seen as an evolutionary byproduct
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One evolutionary development was the shift from fins to hands. Shubin states that “all creatures with limbs, whether those limbs are wings, flippers, or hands, have a common design” (Shubin 30). Despite their different functions, they all have a similar bone structure. While fins are beneficial to fish and other water species for swimming to catch food or escape predators, fins have no use on land. Shubin’s discovery of Titaalik shows that when animals shifted to land, new structures such as wrists and shoulder joints became prevalent (Shubin 40). Later, these structures would develop fingers, which would eventually become the human hand. Because humans and other land animals don’t need to constantly swim to survive, hands are used for grabbing prey. Hands don’t help humans move around, but they are advantageous for our daily functions. Many mammals don’t have hands however, but they have four feet. These feet allow them mobility on land so that they can catch prey. Two of these feet would eventually develop into the two human feet, and the other two would become the human hands.
ReplyDeleteMany primitive species had gill arches that would later become head structures in more developed animals. One early species called Amphioxus showed the first signs of gill arches in which it “pumps water through them to filter out little particles of food” (Shubin 96). The Amphioxus was not yet cephalized, so the gill arches provided a rudimentary function in the organism’s survival. However, the human embryo contains four gill arches that will develop into structures such as jaws, ears, or muscle. The first gill arch becomes the human jaw which also forms some of the ear bones. The second gill arch becomes cartilage and muscle. One bar of cartilage becomes one of the three bones in our middle ear. The third and fourth arches become many of the structures in our throat. These structures allow us to talk and swallow, which are also found in sharks and other fish. (Shubin 91-92, http://www.britannica.com/EBchecked/topic/77543/branchial-arch)
Shubin also describes how jaw bones would eventually lead to the formation of the ear bones. He claims that “the bones at the back of the reptilian jaw got smaller and smaller, until they ultimately lay in the middle ear of mammals” (Shubin 162). This formation is a result of the first gill arch that leads to the formation of both structures. The malleus, incus, and stapes are the three ear bones that allow animals to hear higher frequencies of sound waves than animals with just one ear bone (http://www.britannica.com/EBchecked/topic/42604/ear-bone). This allows animals to quickly detect when they are in danger of predators, and allows them to react more quickly when trying to escape. However, because the ear became more developed at the cost of the jaw, more developed mammals have a weaker jaw that would not let them to catch larger prey as easily. They must depend on other advantages, such as strong arms and legs to catch prey to survive.
Claire Yao (claire.yao521@gmail.com)