How Did The Atmosphere Evolve Into What It Is Today?
FOUR SEPARATE STAGES:
Origin - Primitive Earth was devoid of an atmosphere: because of the Sun's radiation, Earth lost hydrogen, helium, and other hydrogen-containing gases. After several hundreds of thousands of years, the first atmosphere was created by outgassing of the gases trapped in the interior of the early Earth.
Chemical / Pre-biological Era - Extreme volcanism occurred during differentiation – when massive heating and fluid-like motion went on within the mantle. The bulk of the atmosphere was derived from mainly the degassing within Earth's surface early in its history.
Microbial Era - Once photosynthetic organisms evolved, life began to have a major impact on the environment itself. Organisms like the blue-green algae (stromatolite creation) fed off atmospheric carbon dioxide and converted much of it into marine sediments consisting of the shells of sea creatures. Because photosynthetic life reduced the carbon dioxide content of the atmosphere, it also created the some oxygen. For a long time, the oxygen produced did not build up in the atmosphere, since it was taken up by rocks.
Biological Era - Marked by the simultaneous decrease in atmospheric carbon dioxide (CO2) and the increase in oxygen (O2) due to life processes. We need to understand how photosynthesis could have led to maintenance of the ~20% present-day level of O2. The build up of oxygen had three major consequences that we should note here.
--Firstly, Eukaryotic metabolism could only have begun once the level of oxygen had built up to about 0.2%, or ~1% of its present abundance. This must have occurred by ~2 billion years ago, according to the fossil record. Thus, the eukaryotes came about as a consequence of the long, steady, but less efficient earlier photosynthesis carried out by Prokaryotes.
--Secondly, once sufficient oxygen had accumulated in the stratosphere, it was acted on by sunlight to form ozone, which allowed colonization of the land. The first evidence for vascular plant colonization of the land dates back to ~400 million years ago.
--Thirdly, the availability of oxygen enabled a diversification of metabolic pathways, leading to a great increase in efficiency. The bulk of the oxygen formed once life began on the planet, principally through the process of photosynthesis
Planets that contain CO2, or H2O may not be as extraordinary as we may think. They need the will to survive as Earth did -- which is why we are here today. They need to contain some sort of extreme conditions to show that life forms can move past it or, overtime essentially, the extremities will die down and show a world that can possibly sustain life.
"Evolution of the Atmosphere: Composition, Structure and Energy." Evolution of the Atmosphere. Regents of the University of Michigan. Web. 18 Mar. 2015.