Even though every living creature has hands and arms which are used for functions such as grabbing and utilizing various things, their structure is completely different among animals. In the “Getting a Grip” chapter, Neil Shubin talks about how a Scottish surgeon Sir Charles Bell wrote a book about hands. On page 31, a chart is laid out with the different hand/arm structures of different animals. For the animals listed such as birds and bats, their main function is to fly. According to the research done by anatomist Sir Richard Owen about the structure and the order of the bodies, the common plan for almost all of the limbs are “one bone, followed by two bones, then little blobs, then little fingers or toes” (31). With this fact, why does a bird and a bat have different limb structures with different sized bones even though they have the same kind of bones? Or why does a pterosaur and theropod dinosaur have different arrangement with different sized bones even though the function is their limbs is the same: walking? Seals are both land and water animals. Does this have an affect on their bone size and arrangement? If so, how? All of animals listed use their limbs for major functions such as walking or waving it to swim. Should these limb structures be bigger than the human arm arrangement and size because of their constant usage? Humans wave their arms while walking or use them for minor function such as grabbing or working with various things but not for something like using them as “legs”. Why are these limbs smaller and what is the significance?
Chandrika Darbha
shiningstar0393@gmail.com
A bird or a bat requires aerodynamic bone structure. Their bones are porous and therefore can support flight. However, heavier organisms such as humans and elephants require bones that will not break due to the force of gravity. Osteoporosis is analogous to what might occur if a human being would have the bones of a bird or a bat. Due to the activity that we use our bones for on land, porous bones are subject to easy breakages because our behavior has not adapted over the course of evolution to prevent weight from being placed on our bones.
ReplyDeleteThe reason that a bird and a bat do not share exact structures in their wings is because birds and bats evolved at different times. "The ability to fly appears to have evolved separately at least four times: in birds, bats, insects and pterosaurs." (Chatterjee et al, 2007) Although these animals have a common ancestor, their separate evolution patterns make explain why their wing structures are altered. The following are pictures of bat wings and a bird wings respectively:
http://courses.washington.edu/chordate/453photos/skeleton_photos/bat_wing_new.jpg
and
http://people.eku.edu/ritchisong/554images/Bird_wing.jpg
These pictures strongly contrast the finger like protrusions of the bat wing bone compared to the bird wing bone. As the bird evolved earlier than the bat, the bird lacks the defined finger-like bones that the bat has and maintains a more blob-like structure that required less time in evolutionary history to mutate.However, Neil Shubin reminds us that the "despite radical changes in what limbs do and what they look like, this underlying blueprint[composed of one bone, two bones, lotsa blobs]is always present". (30-31)
Furthermore, some limbs are smaller than others because more weight is put on the limbs by the organisms. The greater the amount of activity that utilizes the limb, the better the likelihood that the organism with this limb would survive and reproduce. However, not all limbs evolved for the purpose of solely supporting a given organism. Some crabs such as the fiddler crab are known to have a proportionally large limb for mating. So although most mutations are benefited by the active lifestyle by organisms some mutations are benefited by the reinforcement of that organism to reproduce by the attractiveness of that mutation in the species.
Sources:
http://ukpmc.ac.uk/articles/PMC1973956/
http://courses.washington.edu/chordate/453photos/skeleton_photos/bat_wing_new.jpg
http://people.eku.edu/ritchisong/554images/Bird_wing.jpg
Your Inner Fish
Yekaterina Khavkhalyuk (kittykatx93x@yahoo.com)