In Chapter 7, Shubin talks about building bodies with different types of cells and division of labour, and relates it to the early experiments of H.P.V.Wilson on sponges and their reassembly after being passed through a sieve. He also deals with why there are bodies, noting that for the first 3.5 billion years of evolution there were only microbes and then suddenly bodies appeared.What are some of the reasons that ‘bodies’ might have developed in the first place? Include any environmental conditions that might have favored their evolution.
-Adnan Jahan
(adnanjahan@gmail.com)
Neil Shubin describes the sudden emergence of bodies after so much history without them as the "perfect storm" (Shubin 135) because previously, animals didn't have the energy source in order to support a body with. That is, bodies are very expensive in their energy requirements, especially as they grow larger and larger.
ReplyDeleteThe main environmental condition that lead to the development of bodies in animals was the increase in oxygen levels in the environment. As we have learned previously this year, oxygen is one of the main components required for aerobic cellular respiration, which generates large amounts of ATP to be used by the body for functions such as movement and active transport across a membrane when moving against the concentration gradient (Campbell & Reece 176). Oxygen is important to this form of cellular respiration because it is the highly electronegative molecule "downhill" in the electron transport chain during the process of chemiosmosis; NADH and FADH2 carry high-energy electrons to the electron transport chain, which is in the membrane of the mitochondria. Ubiquinone and cytochrome c shuttle electrons between large protein complexes, and as this transfer occurs, protons are pumped from the mitochondrial matrix into the intermembrane space. This generates a proton-motive force which allows for the protons to flow back down their gradient through the ATP synthase, which phosphorylates ADP to form ATP (Campbell & Reece 175). Water is also formed through this process.
Having a body became an evolutionary advantage, something we can attribute to the theme of evolution--"a process that results in heritable changes in a population spread over many generations" (http://www.talkorigins.org/faqs/evolution-definition.html). As we learned previously about the process of natural selection, over time traits that are seen as beneficial to a certain species or population are maintained over time (http://evolution.berkeley.edu/evolibrary/article/evo_25); in this case, the more advantageous trait would be having a body. This is because, though a body may have more energy requirements, the benefits it offers to an animal may make up for the energy it must expend to maintain the body. Some of these advantages are described by Shubin--the body allows for the animal to get "big, [move] around, and [develop] new organs that helped them sense, eat, and digest their world" (Shubin 119). Thus, bodies developed in order to aid animals in defense and competition in a world where everything seemed to be getting bigger and stronger. This reminds me of something we learned earlier this year--colonial vs. multicellular. Unlike colonial organisms, animals with bodies have cells that "work together to make a greater whole" (Shubin 118). This cooperation, once again, allows for our bodies to have many more functions than those that were possible in colonial or single-celled organisms.
Sources:
Your Inner Fish
Campbell & Reece
http://evolution.berkeley.edu/evolibrary/article/evo_25
http://www.talkorigins.org/faqs/evolution-definition.html
Kathy Li, kathy2132@gmail.com
Kathy makes a great point by saying the first environmental pressure is because the presence and concentration of oxygen increased allowing for cellular respiration in the individual cells. I agree with everything Kathy has said, but I believe she is missing one of the simplest reasons why we have bodies. Because of cellular respiration and the increase in oxygen for ATP to be made, energy was simply an easily accessible source for the unicellular organisms. Because of the easily accessible source of energy these cells had, they had the opportunity to undergo mitosis and divide and expand their cell size. This brings in the theme of energy transfer. So much food and energy requirement nutrients were found in the environment that the cell’s metabolism drastically increased allowing them to catabolize molecules much more and faster providing the cell with extra cellular molecules that the cell had nothing else to do but get bigger and larger. The molecules that were formed were most likely proteins and phospholipids to increase cell membrane first to allow more growth inside the cell. I believe that because the cells were able to divide and grow as much they want, they were able to get bigger and bigger and ultimately, develop bodies. Just like humans you see today, if we have the food available, we will eat and get bigger because we have the energy source we need to grow. You can argue that the same exact environmental condition billions of years ago that caused this evolutionary change is still going on right now. However, that is just the big picture on why bodies developed.
ReplyDeleteTo get to the details, it all started with prokaryotes only having the nucleus, cytoplasm, and ribosomes. Then by endosymbiosis, “a process in which a unicellular organism engulf another cell, which lives within the host cell and ultimately becomes an organelle in the host cell”, the unicellular prokaryote became what we call the eukaryote (1). Through this process, the mitochondria and other organelles were formed inside the cell. This clearly demonstrates the theme of evolution. The individual prokaryote with no organelles performed endocytosis which is the “cellular uptake of biological molecules and particulate matter” to gather food and energy at first, but then by chance it happened to absorb another cell (1). Soon as the cell developed into an organelle like the mitochondria, it was a selective advantage for the cell to have this organelle and as it divided, the cell survived better and easier. Endocytosis brought more “stuff” inside the cell and increased the cytoplasmic volume which further enhanced the cell’s ability to grow. Pinocytosis, a type of endocytosis, was able to bring more fluids into the cell to help the future organelles settle and stabilize in the cytoplasm (2). As the cell was able to grow, it is very possible that when the cell divided it stayed in colonies to survive together. Although the research isn’t yet sufficient enough to prove this idea, evolution of multicellular organisms could have originated from colonial organisms (3). The reason why colonial is not considered to be multicellular is because they lack certain requirements to be considered a multicellular organism. One of them is specialization.
Incorporating the theme of evolution. By chance and variation, a certain colonial organism could have had one or two cells only specializing in certain aspects inside the cell like possibly only the function of the mitochondria. This eventually became a selective advantage for this organism because this only “mistake” cell starting providing the colonial organism with all the necessary energy to function. Then began a chain of “mistake” cells that began to specialize in a specific function that gave rise to the multicellular organisms we see today. All cells originate from the same way, but by development the cells branch off to do their own specific duty to enhance the performance of the organism as a whole by having a bunch of cells focusing on one area. As the number of cells increased in that area, the tissue was born, then the organ, then the organ system. Eventually, humans were born with the most sophisticated system of specialized cells that all began with endosymbiosis.
ReplyDelete1. Campbell
2. http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/E/Endocytosis.html
3. http://creation.com/multicellularity
Benny Jeong
bennyjeong218@gmail.com