From the polar bear to the turtle to the human, we're all related to some degree (179). Shubin discusses the "pattern of relatedness" in the organisms of the Earth and the commonalities we share even with the strangest of animals. For instance, polar bears and humans both share hair, mammary glands, four limbs, a neck, two eyes, etc. (179). To conclude, the polar bear would be more closely related to the human than the turtle or the fish who shares less common characteristics (179-180). This is how ancestry and relatedness is deciphered by paleontologists or other scientists.
Understanding this concept, if Tiktaalik shares many more structural characteristics than the average fish, how would they relate to the human as far as ancestry goes? And, if Tiktaalik was the bridge between limbless organisms and limbed organisms, then why do we see more fish without these appendages today rather than fish like Tiktaalik? What caused fish without the appendages that Tiktaalik had to prevail in the oceans?
Sonia Doshi soniadoshi7@gmail.com
Since fish evolved, they evolved in multiple ways depending on the need of the organism because of the environment. Obviously, the ancient fish evolved into the modern fish and on the other side humans. Tiktaalik was one of the evolutionary steps to humans as it was the bridge from fish to human. The evolution of limbs quite simply, “enabled Earth's animal life to crawl from water to land” (http://news.nationalgeographic.com/news/2004/04/0401_040401_tetrapodfossil.html).
ReplyDeleteThere are more fish that do not have limbs than that do (like Tiktaalik) because of the effectiveness of the adaptations is specific environments. The limbs must have been inefficient in water therefore killing off many of the organisms that live in water that have limbs. On the other hand, the limbs were very useful when it came to land so any organisms that migrated to land were able to survive. Therefore the prominence of the limbs introduced by the Tiktaalik were only kept by terrestrial animals. This is ironic because Tiktaalik lived in water and did not die out before moving to land even though it is clear that limbs do not work very well with aquatic environments.
Limbs in the ocean were not advantageous because of the properties of water. The fastest was to move in water is to be aerodynamic (which extraneous appendages do not add to) and the ability to use the movement of water to one's ability. If the organism is crawling on the floor of the ocean, then they are not able to move as fast as a swimming organism because they are using buoyancy and water tension to their advantage. Also, few other organisms live on the sea floor, so there are limited supplies for Tiktaalik to survive if it remains to have limbs.
Jackie James
(jackie.james@comcast.net)
This comment has been removed by the author.
ReplyDeleteJackie makes a correct point in saying that terrestrial limbs are not suited for marine organisms because of gravitational differences between air and land, and therefore do not prove to be a selective advantage for any current species of fish today. To elaborate on Sonia’s question of why more modern fish are unlimbed as opposed to limbed like Tiktaalik, however, consider that Tiktaalik had eventually evolved into the four-limbed Acanthostega and Ichthyostega group of organisms which both thrived as the first evolutionary amphibians (Source 1). Previous research seems to suggest that Icthyostega is the more fishlike of the two, and did not evolve as complex of limbs as Acanthostega did, so perhaps this deviation in genetic structures led certain individuals in Icthyostega (which was already more aquatic-based in environment that Acanthostega) to return back to their dependence on marine environments and evolve into more modern species of fish without fully-developed limbs, whereas Acanthostega descendants were more likely to continue thriving on land (Source 1).
ReplyDelete(continued) Even though primitive fish and Tiktaalik were considered “all closer relatives of one another, more like cousins to different degrees rather than direct ancestors (Shubin 209, Afterword), these fish likely had even weaker limb/fin structure than even Tiktaalik and its amphibian counterparts, because previous research reveals that while “is true that the front fins [of Tiktaalik] are quite robust...The bad news however is that the backbone is very weak at the hind (pelvic) end and the pelvic fin is extremely small”, which still presented problems in land mobility that other non-limbed fish would’ve had even more difficulty overcoming (Source 2). Thus, while it was near impossible for other fish to have as much terrestrial success as Tiktaalik, the converse fact remains that Tiktaalik would also have had more difficulty competing in a deeper marine environment because of those very limbs; fish without limbs tend to navigate the undulating pattern of wave currents much easier than limbed fish would (Source 2). Moreover, to elaborate on Shreeraj’s points about fins being much easier to use than limbs for water navigation, Neil Shubin mentioned earlier on in “Inner Fish” that the limbs found in primitive Acanthostega gunnari, an organism that would’ve been an amphibian counterpart of Tiktaalik possessed a “limb that was shaped like a flipper, almost like that of a seal. This suggested to [the scientist] that the earliest limbs arose to help animals swim, not walk” (Shubin 35). Thus, even if other fish did have limb-like fins, they were already vastly different to those limbs of Tiktaalik because the structure of the parts would wholly depend on the functions of walking (where Tiktaalik used strong front fins to support and push forward the rest of its body on terrestrial shores) and swimming (obviously more useful in a marine environment.)
ReplyDeleteSources:
1. http://www.earthhistory.org.uk/technical-issues/tiktaalik-roseae
2. http://www.create.ab.ca/tiktaalik-and-fellow-fish-fossils/
3. Neil Shubin, Afterword and Chapter Two
Christine Lin
choco_cat11@comcast.net
To go off of what Jackie had said, limbs are much heavier than fins. Fins in fish contain lipids which are lighter than bone. Because of this reduction in weight, fish with bones filled lipids "will reduce the negative buoyancy per volume of bone" (http://fishanatomy.net/webpages/Facts/fins/fins.html). thus, fins are lighter. Because fins are lighter, swimming is a lot easier. As seen in normal everyday life when one is in a swimming pool, it is much harder to run in water than swim in water. The aerodynamics of a standing human in a pool is much less than a swimming man because there is more surface area in the pool that is touching the "current". Similarly, in fish, because there are fins, there is less surface area touching the "current". The aerodynamics in a fin is much better than that in limbs. Thus, speed is greater and the ease of swimming is much easier. Also, as discussed in class, most adaptations occur because it allows the organism to live an easier life with less energy used. Swimming using limbs along a sea floor takes more energy than swimming with fins with the current in open water.
ReplyDeleteAlso, to compare Tiktaalik to the modern humans and fish, it is a bridge between the two. However, many fish are present that contain limbs. They aren’t the most effective as Jackie stated. But also, it is mainly because limbs are heavier than fins. Thus, fish with limbs are rare. As for Tiktaalik being a link between humans and fish, there are many similarities. Tiktaalik contains a neck that allows the head to turn just as a human does. However, because the fish is in water, the need for this isn’t necessary as many animals in the ocean do not contain this. Sea turtles for example contain a neck that allows the head to turn a large amount. They also contain fins that help the turtle swim. A normal land turtle contains actual feet. As seen in the turtles, fins are an advantage in water and feet are an advantage on land. Because of this advantage, the traits stayed over generations of change. Similarly, Tiktaalik contained feet; however, in water. Thus, Tiktaalik was unable to survive as it could not swim as well as the surrounding organisms. Although it seems unreasonable, it is true that Tiktaalik couldn’t swim as fast as normal fish because of the absence of fins.
Sources:
http://fishanatomy.net/webpages/Facts/fins/fins.html
Shreeraj Patel
shreeraj.patel1@gmail.com