Chapter 2 Getting a Grip

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  1. 1. Describe the “pattern” to the skeleton of the human arm that was discovered by Sir Richard Owen in the mid-1800s. Relate this pattern to his idea of exceptional similarities.

    The pattern that Sir Richard Owen discovered in the mid-1800’s was common throughout all the observed bones of mammal limbs. The “one bone-two bones-lotsa blobs-digits pattern” simply refers to the position of the bones, starting with the humerus in the upper arm, followed by radius and ulna in the forearm, then several wrist bones, and finally ending with the digits (Shubin 32). The same pattern applies to the legs – femur, tibia and fibula, cluster of small bones, and phalanges. This relates to his idea of exceptional similarities which states that “all creatures with limbs, whether those limbs are wings, flippers, or hands, have a common design” (Shubin 30). In other words, this pattern was the “blueprint” for the structure and placement of mammal limbs. He continued to look at skulls and backbones and considered the same thing – there is a fundamental design in the skeleton of all animals.

    2. How did Charles Darwin’s theory explain these similarities that were observed by Owen?

    Charles Darwin said that the reason the reason a bat’s wing and a human’s arm share a common skeletal pattern is because they shared a common ancestor at one point in time. This reasoning can be applied to anything that has limbs.

    3. What did further examination of Tiktaalik’s fins reveal about the creature and its’ lifestyle?

    After further examination of Tiktaalik’s fins, it was decided that the 375-million-year-old fish had wrists. Similar to a human, its elbows could bend, and with its wrists flat on the ground, it had the capability to do a “push-up”. This type of movement for a fish was important so that it was able to navigate the bottom and shallows of streams or ponds, and even [on the mudflats along the banks]” (Shubin 40).

    • Thank you Lakshmi. Your replies are thorough and well constructed. You have chosen well with the quotes included to support your responses.
      We see our classroom learning reflected in this chapter regarding homologies, and Darwin. ML

  2. It seems as though even if all animals have a fundamentally similar body plan, they have evolved to specialize certain parts to an extent. If each part of a body plan is specialized by a different species, the possibilities of organisms are enormous. This is the same homology that makes whale flippers related to human hands.

    • Yes, so if a catastrophe causes another major extinction, it’s difficult to predict what kind of animal body plans will emerge and be dominant next. ML

  3. It’s remarkable that Tiktaalik was supposedly able to navigate the banks of rivers. This suggests the origins of terrestrial animals, which provides yet further evidence for the relationship between humans and Tiktaalik. Such an assumption makes it seem more than likely that a series of accumulated mutations could have resulted in a fully amphibious descendant of Tiktaalik.

  4. Morgan Moroi

     /  May 7, 2012

    I agree with Arman… The mere fact that Tiktaalik had the ability to “walk” on the bottoms of streams or ponds, solidifies that Tiktaalik serves as a transition between fish and amphibians.
    In addition, this reminded me of the type of fish that I researched for my project earlier in the year – dipnoi. They also had the ability to move through water using “leg-like” appendages. Could this suggest that they are very closely related in regards to evolution? Absolutely. In fact, Tiktaalik and Dipnoi share the same class.


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