Exoplanet discussion- Part 1: atmosphere

Let's talk about exoplanets. There is a multitude of questions that may come up when it comes down to it.  Let's assume we're talking about an earthlike planet
1) What if gasses are distributed or concentrated differently?
The atmosphere may have higher or lower amounts of gasses than here on Earth. For example, there may be twice as much oxygen, or 25% of the carbon as compared to Earth. Then we could also explore other gasses such as nitrogen, hydrogen, helium, methane, etc.
A planet with twice as much oxygen as Earth would likely have life that's larger than here one Earth. We have evidence for this. In prehistoric times, Earth went through a period of time in which our atmosphere contained large amounts of oxygen. This then caused animals' lungs to grow larger to contain more oxygen, which is pretty interesting since you may have thought otherwise. Obviously, a planet with less oxygen than Earth would have smaller animals. You may be wondering how humans from Earth arriving there would be effected. That depends on how low or high the oxygen levels are compared to Earth. With less oxygen, of course it would have adverse effects and may even cause brain damage in extreme cases. With more oxygen, the less obvious effects may occur. One of these include abnormally high red blood cells. Another, yet interesting, effect that may occur is that the high oxygen levels may have positive effects for people with breathing problems. How about effects on the ecosystem? One example of such an effect would be on animal life, which may be more abundant. Also, if there are any bodies of water, there may be more fish if the water has more oxygen. A planet with more carbon could have adverse effects if we landed there. Rain would be more acidic, but that doesn't have to mean aquatic life would suffer. In fact, here on Earth aquatic life including coral reefs have been found to thrive just as well under acidic water. Of course, more carbon= an atmosphere that insulates heat better, which may make a world habitable, or not. Supposedly, excess amounts of CO2 may cause plants to deteriorate. So the debate on climate change here on Earth helps to gather such interesting information.
More Methane on a planet would be like carbon. Like carbon, Methane is a greenhouse gas. Scientists think it may have contributed Earth's past climate. Interestingly, if you had a planet just like Earth but much hotter, & this planet had an ice age, the role of methane and/or carbon would come in handy. Methane has the ability to trap ten times as much heat as CO2, which traps a few times as much heat as oxygen. Methane is a very abundant gas on Europa, which features geography much like Earth. Unfortunately, Europa isn't suited for life. However, it is believed that Europa may have geysers. If that's the case, there may be hotspots where life could cling on. Exoplanets will turn out to be much more interesting than one would likely expect.
High amounts of gasses, especially CO2 & methane, could be a problem when it comes to humans industrializing a world, as a warming atmosphere fills up with pollutants more easily as opposed to a cool atmosphere. Ironically, Carbon itself is known to be an air purifier.