In describing his encounter with the "missing link" in evolutionary history, Neil Shubin identifies an extraordinary feature in
Tiktaalik. This organism had a wrist bone in its fins, a feature seemingly useless for a fish to have. However, a close look at the structures of various joints revealed that the wrist enabled
Tiktaalik to perform push-ups (Crazy!). Neil Shubin theorizes that "[f]ins capable of supporting the body would have been very helpful indeed for a fish that needed to maneuver in [various environments]", allowing them to walk on land (Shubin, 40).
Why was the transition from water to land necessary?
Consider the theme of Evolution: What were the selective advantages that terrestrial organisms (such as mammals) had over organisms that live in water?
Include in your answer terms from Unit Thirteen, Invertebrates and Vertebrates: Predation, Internal/External Fertilization, Internal/External Development, Amniotic Egg, etc.
(Keigo Tanaka; tanakarus3@hotmail.com)
Tiktaalik symbolizes the missing link between water and terrestrial organisms. With fins, as well as the beginnings of shoulder and wrist joints, Tiktaalik “broke down the distinction between these two different kinds of animal” (23). This distinction became necessary over time because terrestrial life offered several advantages that water life could not: more oxygen, ideal levels of competition, and many new adaptations.
ReplyDeleteOne reason terrestrial animals may have left their aquatic environments is for more oxygen (http://wiki.answers.com/Q/What_advantages_over_an_aquatic_organism_
does_a_terrestrial_organism_have_with_regard_to_obtaining_oxygen_for_respiration). Air offers a much higher concentration of oxygen than water, and thus acquiring ATP through respiration ultimately would be easier on land. Also, new terrestrial animals were able to exploit the benefits of newly discovered habitats: more prey with fewer predators. Because few, if any animals, had inhabited the land prior to these new terrestrial animals, few predators were around to attack. Likewise, few competitors were around to exploit the plants and other food sources that had already long inhabited land. New land animals experienced the best of both words, enjoying a plethora of food with few other species to stand in their way.
But beyond these basic, short-term benefits to life on land, terrestrial living offered the opportunity to develop many new adaptations that would not arise aquatically. Shubin discusses many of these adaptations extensively through Tiktaalik, for even though Tiktaalik was not terrestrial himself, he exhibited many of the adaptations that lead to land animals. For example, Tiktaalik exhibited a bendable elbow and wrist that allowed him to maneuver the ocean floor and would later allow terrestrial animals to navigate land (39). Eyes on top of the head allowed for a similar advantage in navigation (24). Navigation of land would allow for exploration of new food sources. Other key adaptations include internal fertilization and development. While some land animals, such as birds and reptiles, maintained external development, internal fertilization became a key advancement for land animals. Internal fertilization provides more accurate fertilization and the mother does not need to expel excessive amounts of energy to release thousands of eggs. The amniotic egg, seen in reptiles, is an example of terrestrial internal fertilization and external development. The four membranes of the egg surround the embryo, forming an ideal mini environment for the embryo to grow, leading to reproduction that was independent of aquatic habitats (Campbell, 714). Furthermore, the advance of internal development allowed for more protection and nutrient exchange from the mother, in turn leading to a higher survival rate and prolonged lifespan. Without the transition to land, these adaptations would not have developed.
Many other adaptations developed from terrestrial life, many giving these animals a selective advantage over aquatic animals. Life on land allowed animals to obtain more oxygen, temporarily alleviate pressures from competition, and develop into more complex beings, or mammals.
Sami Kopinsky
(sami_kopinsky@yahoo.com)
In addition the reasons that Sami mentioned, such as predation and resources on land, there are many other reasons why animals colonized land, such as changes in their body structure. Vertebrae evolved from fishes with lungs (Klappenbach). It is estimated that vertebrates evolved approximately 500 million years ago, which is not too long before animals began colonizing land (Farabee). When animals colonized land, they needed to be able to with stand the force of gravity; therefore, having a backbone would have been advantageous in helping an animal to balance and support its weight (Klappenbach). Animals with backbones evolved from fishes with lungs, so it makes sense that these animals would eventually colonize land because having a backbone in water would not be necessary or advantageous.
ReplyDeleteIn addition to what Sami mentioned about internal and external fertilization, it is important to note that the external fertilization and development is commonly used to produce many organisms at one time, mainly r-selected species, many of which die at a young age. On the other hand, an organism that reproduces with internal fertilization and internal development is more likely to produce a fewer number of offspring (Bailey). Organisms that use internal development are able to care for their young more effectively, and therefore the young have a higher chance of survival. Therefore, since the transition from water to land caused a transition from external fertilization and development to internal fertilization and development, this transition also caused a shift from r-selected species to more k-selected species. In terms of evolution, K-selected species have more of an advantageous because they have more nourishment and care in order to be able to survive and reproduce.
Sources:
http://www2.estrellamountain.edu/faculty/farabee/biobk/BioBookPaleo3.html
http://animals.about.com/od/amphibians/ss/landtowater.htm
http://biology.about.com/od/genetics/a/aa040805a.htm
Marissa Lobl (marissa.lobl@gmail.com)