Organic evolution and biological change

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),