Essay on "Organic Evolution"

Essay 12 pages (4338 words) Sources: 10

[EXCERPT] . . . .

Organic Evolution

Please discuss the pre-biotic conditions on planet earth. Why did it take approximately one half billion years before the earliest bacteria-like life evolved? Why did the formation of oxygen by photosynthesizers make such a difference on the planet? Specifically, why does it appear that the aerobic metabolic pathway is a mirror image of the photosynthetic pathway? What would have happened to this system if oxygen had been present on earth 4 billion years ago?

Scientists believe that about 5 million years ago, the Solar System was filled with a plethora of hot gases and dust, swirling around a hot core. They think that once the core approached about 1 million degrees, the physics and chemical properties caused the gases to coalesce, forming the sun. During this time, there were millions and millions of asteroids. As these asteroids collided with one other, some combined and as their mass increased, gravity pulled more and more particles and debris in, and the planetoids became larger and larger until the planets of the solar system were formed. This was a process known as accreation, and over hundreds of millions of years, the solar system formed -- the continual bombarding of asteroids changing the planets, forming the rings of Saturn, and the landscapes of others, including the moons -- which were just smaller planetoids caught in the gravitational pull of the planet (Palmer, 2003).

The constant bombardment of these asteroids did two things: first, it released an enormous amount of energy onto the earth's crust, causing it to melt in places. Second, by opening up chasms, radioactivity was released and caused mega-volcanoes to release molten rock and gases, which in turn reacted to even more of the asteroid bombardment. H2) was released from the meteorites and the crust, rising as a gas into the atmosphere, where it combined with CO2 and other cases, and formed dense clouds above the earth. The clouds acted as a kind of reflective shield above the planet, keeping solar heat from penetrating to the surface and evaporating any residual moisture. With this shield, a primitive atmosphere was formed, further protecting the planet from continual bombardment at the previous levels. Thus, with fewer impacts, a dense cloud shield, and the combination of gases, the earth began to cool a bit and rain pelted down cooling the molten rock from the volcanoes, thus creating lakes and the great oceans. A combination of heat, water, and wind then began to soften and eventually cause more of the geologic features familiar today (Fortey, 1999).

For years, scientists through that the early Earth had a reducing atmosphere in which molecules saturated with hydrogen atoms reduced other molecules. Now, most think that the early Earth was full of oxidants like CO2 and N2 -- neutral and does not permit organic chemistry to occur. There are several theories about how life originated on earth, some more credible than others. We know that liquid water is essential in the biochemistry of living things, allowing a medium for the transport of molecules, but where might this have existed and been able to combine with other chemicals to form carbon-based life, and simple plants which would create other chemical reactions through photosynthesis?

Thermal Vents -- One theory believes that life originated deep in the ocean around hydrothermal vents. The vents release hot cases from earth's core, sometimes in excess of 600 degrees. Primitive bacteria, small worms and crabs survive in this environment today, suggesting that life may have begun here and then moved upward. Supporters of this theory believe that the organic molecules are formed in a gradient layer between the hot vent water and the ocean cold water (Van Dover, 2000).

Panspermia -- Swedish chemist Svente Arrhenius developed a theory stating that life did not originate on earth, but elsewhere in the universe. Primitive cellular structures arrive through meteorite activity and since they were protected by being inside the meteorites, once they hit the earth the cells could have evolved and restructured (Hoyle and Wickramasinghe, 2000).

Frozen Ocean -- Scientist Jeffery Bada of the Scripps Institution proposes that only the top 300 meters of the ocean would freeze during a thick cloud bank on the surface. The ice would shield prebiotic chemicals by preventing UV light through and allowing a safe place for life to incubate. The cooler temperatures and relative safety then encouraged organic chemical reactions (Bada and Wills, 2000),

It was essential though that primitive cells be able to develop into molecules that evolved a new strategy to use sunlight as an energy source. These cells took in sunlight, used the CO2 and H2O as raw materials produced organic molecules (carbohydrates). O2 was released as a waste product of photosynthesis, first bonding with limestone, iron and other minerals. Finally, though, once these minerals oxidized, O2 began to ooze into the atmosphere. As this O2 rose higher into the atmosphere, it changed chemically to form ozone, which was critical for development of carbon-based life b3ecause it absorbs a significant amount of ultraviolet radiation and allowed cells to colonize the ocean and eventually land. Without O2 and the resulting ozone layer, continued bombardment of the surface by intense UV light would have caused unsustainable levels of mutation in exposed cells. More than likely, without ozone, cellular structures would never have become stable enough to evolve into higher forms. Photosynthesis had another, critical impact. Oxygen was toxic; and probably much life on earth died out as its levels rose too high in what is termed the oxygen catastrophe. This occurred about 2,400 million years ago. While photosynthesis was producing oxygen both before and after this "event," the difference was that before the catastrophe, organic matter and dissolved iron chemically captured any free oxygen. The Great Oxygenation Event (GOE) was the point when these minerals became saturated and could hold no more free O2; and the excess began accumulating in the atmosphere (Chaisson, 2005).

Thus, photosynthesis takes in CO2 and produces O2, while organic metabolism takes in O2 and respirates out CO2. Photosynthesis requires carbon dioxide and water, in the presence of sunlight, and results in oxygen and glucose. Cellular respiration requires oxygen and glucose, and forms the products of carbon dioxide, water, and ATP (energy). The mirror image of the process is likely synergism at its most basic -- without one, the other would be suplerflous -- but in combination they support the essence of life, and a way for the early earth to become populated with not only plants, but the precurors to animal life (Willis, 2001).

REFERENCES

Bada and Wills. (2000). The Spark of LIfe. New York: Basic Books.

Chaisson, E. (2005, June). Early Cells. Retrieved October 2010, from Tufts University: http://www.tufts.edu/as/wright_center/cosmic_evolution/docs/text/text_bio_1.html

Fortey, R. (1999). Life:A natural history of the First Four Billion Years of Life on Earth. New York: Vintage.

Hoyle and Wickramasinghe. (2000). Astronomical Origins of Life. New York: Springer.

Palmer, D. (2003). Prehistory Past Revealed: The Four Billion Year History of Life on Earth. Los Angeles: University of California Press.

Van Dover, C. (2000). The Ecology of Deep-Sea Hydrothermal Vents. Princeton: Princeton University Press.

Willis, B. (2001, July). Photsynthesis and Cellular Respiration. Retrieved October 2010, from Worsleyschool: http://www.worsleyschool.net/science/files/photosynthesis/page.html

Biologists have found that the majority of genetic code in higher animals appears to serve no function. These large sequences appear to be the result of mutations that led to insertions. Logically, there should be a cost to having extraneous DNA. What is this cost? Why does natural selection not act by favoring organisms without these extra sequences of nucleotides? How do you interpret the data on bacteria that tend to have small genomes and lower amounts of "Junk DNA"? Do extra copies of genes offer organisms any advantages? (hint: in your answer discuss Hox-gene complexes and their importance in the evolution of animal).

DNA is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms and some viruses. The main role of DNA molecules is the long-term storage of information. DNA is often compared to a set of blueprints or a recipe, or a code, since it contains the instructions needed to construct other components of cells, such as proteins and RNA molecules. The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in regulating the use of this genetic information. Chemically, DNA consists of two long polymers of simple units called nucleotides, with backbones made of sugars and phosphate groups joined by ester bonds. These two strands run in opposite directions to each other and are therefore anti-parallel. Attached to each sugar is one of four types of molecules called bases. It is the sequence of these four bases along the backbone that encodes information. This information is read using the genetic code, which specifies the sequence of the amino acids within proteins. The code is read by copying stretches of DNA into the related nucleic acid RNA, in a process called transcription… READ MORE

How to Reference "Organic Evolution" Essay in a Bibliography

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