In chapter seven, Shubin discusses several theories about the appearance of bodies on earth. One theory is that the increased amount of oxygen contributed to the rise of bodies, and another theory is that predation contributed to microbes getting big and forming bodies. Thinking about the theme of evolution, how could the rising level of oxygen have contributed to the rise of bodies? What else happened as result of rising oxygen levels on earth that supports this theory? Why might predation have caused the formation of bodies? What specific adaptions to predation supports this?
Marissa Lobl , marissa.lobl@gmail.com
Oxygen levels on ancient earth were relatively low, but then “roughly a billion years ago, the amount of oxygen increased dramatically and has stayed relatively high ever since” (Shubin 137). Oxygen is needed to drive metabolic processes for cellular respiration in the mitochondria so that the body will have enough energy to survive and reproduce. When there is a lack of oxygen, cells will go through anaerobic respiration and fermentation. This yields to only two molecules of ATP that were generated from glycolysis for energy, which is enough for extremely tiny one celled or several celled organisms. However, it is not sufficient energy for larger organisms such as mammals or reptiles to survive with. When more oxygen is present, cells will go through aerobic respiration, where the 2 ATP, 2 Pyruvate, and 2NADH molecules will move on to the citric acid cycle. In the citric acid cycle, 6 NADH, 2 FADH, and 2 more ATP molecules are generated, and will move on to the electron transport chain and chemiosmosis. During chemiosmosis, H+ ions flow back down their gradient into the mitochondrial matrix from the intermembrane space (Campbell 175). The movement of H+ ions phosphorylates ADP to form ATP. From chemiosmosis, about 32 to 34 molecules of ATP are generated. All together, aerobic respiration can form 36 to 38 molecules of ATP, which provides much more energy for larger bodies than the 2 molecules generated from glycolysis in anaerobic respiration (Campbell 176). Body size also relates to the amount of oxygen that can be transported around the body by the circulatory system. Animals have evolved from fish with two chambered hearts, to reptiles with three chambered hearts, to mammals with four chambered hearts, which contain two atria and two ventricles. Larger animals have a more intricate circulatory system, in which the oxygenated blood remains separate from the oxygen poor blood. When more oxygen became available, organisms could easily inhale enough oxygen for their heart to pump the oxygenated blood to the cells for cellular respiration.
ReplyDeleteShubin claims that, “When predators develop new ways of eating, prey develop new ways f avoiding that fate” (Shubin 136). The earliest microbes survived by engulfing other microbes as food and energy. The larger microbes would have an easier time eating smaller microbes, while it would be harder for smaller microbes to engulf microbes larger than itself. Thus, a larger body size became a selective advantage of these single celled organisms. Over time, cells clumped together to increase the size against predators, as seen from Martin Boraas experiment with algae (Shubin 136). When the predator was introduced, the single celled alga clumped together, eventually forming clumps of eight cells. However, when the predator was removed, the algae continued to reproduce in clumps of eight cells, showing the evolution of the generations of the original one celled alga. Eventually, the abundance of oxygen on earth allowed for organisms to migrate out of water and onto land. Pierre Legagneux and other scientists published on the Oxford Journal that, “Among the processes known to promote emigration are competition avoidance, small size or low quality of the natal habitat, inbreeding avoidance, high prevalence of parasites, body condition, or high predation pressure” (http://beheco.oxfordjournals.org/content/20/1/186.full). High levels of competition for survival caused generations of organisms to survive in terrestrial regions of earth in order to avoid predation, or to find new advantages for competition on land. Those with larger bodies had a better chance of surviving on land because they had legs or feet for faster movement that the single celled organisms that only had flagella for slower swimming. This advantage allows individuals to have the benefit of catching prey quicker, and escaping predators more easily.
Claire Yao (claire.yao521@gmail.com)
Oxygen is a very important and vital necessity for life on land. As Claire mentioned above, Oxygen is needed in order to continue driving certain metabolic processes for cellular respiration in the mitochondria, which produces enough energy in the form of ATP for the animal to survive. If there is no oxygen during cellular respiration, then cells can go through anaerobic respiration in which very little energy (ATP) is produced. This small amount of energy produced by insufficient amounts of oxygen is not enough for terrestrial animals to survive.
ReplyDeleteBillions and billions of years ago, and for billions of years before that, the amount of oxygen in the atmosphere was very scarce. This scarcity of oxygen is the reason why there were no terrestrial animals/mammals on earth at this time. Up to a point, most animals lived underwater because the amount of oxygen under water was much greater than that of the air. As oxygen in the atmosphere suddenly increased (which was proved by the chemistry of rocks, according to Shubin), the now sufficient amount of oxygen in the air led to the evolution of species over millions of years and generations so that they could move from their mostly water-based habitats to terrestrial habitats. All of the oxygen in the atmosphere could be used by animals to produce the energy that they needed to survive on land and reproduce and account for the costliness of animals with bodies that need lots of energy (ATP) to survive.
As evolution brought the transition of bodies on land because of sufficient amounts of oxygen in the atmosphere, the body sizes of animals also transitioned from smaller bodies to larger bodies. Larger body sizes can usually only survive on land and not water because they are very costly and require lots of energy, which requires lots of oxygen that only the air can provide. Due to predation, evolution has brought change from small body sizes to large body sizes. Animals with a smaller body size are more susceptible to predation. Also because of predation, some animals have adapted by having their bodies protected by a special outer membrane, such as a shell or skin. An example of an animal that has a hardened shell on the outside is an armadillo. An armadillo's shell is made of plates of dermal bone that are useful for protection against predation. This shell is protective against animals that try to eat the armadillo or physically attack the armadillo. Large bodied animals also have an advantage over small bodied animals because they can eat smaller animals as prey. Larger animals also have more complex bodily systems that have more detailed ways of transferring and utilizing oxygen efficiently throughout the body.
Sujin Ko (sujinko93@gmail.com)
Sources: Shubin
Campbell
http://en.wikipedia.org/wiki/Armadillo