The focus of my question pertains to the natural chemical and biological mechanisms which evolved the atmosphere on Earth as we have known it up to modern times (excluding human activity), and how it might inform expectations around the occurrence of other "Earth-like" planets yet to be discovered.
Is the quantity of nitrogen stable? If so, where did it come from and why isn't it continuing to increase? Why is it molecular N2 and not, for example, ammonia as we see on other planets or large moons in our solar system? I suppose this question could be extended to include Venus - why it has its supply of nitrogen (and why it is several times that of Earth). One theory on the emergence of life on Earth states that amino acids formed from a "chemical soup" which included ammonia, which can only be found today in trace amounts.
What about the quantity of oxygen? Oxygen is one of the most abundant elements on Earth, most of it in the rocks of the crust and mantle. Again, excluding any effects of human activity, and aside from the obvious effects of life, what affects the quantity of oxygen in the atmosphere that brought it to and/or maintains it at modern levels?
Why are the partial pressures of these principle gases at the values as we currently know them and not some other values? Excluding the effects of human activity, is Earth's atmosphere continuing to evolve? If so, in what way, and if not, is it because of some sort of equilibrium, and if that is so, what are the processes involved?
Hypothetically: if we found another rocky planet of approximately Earth mass/surface gravity orbiting a host star similar to our sun at a distance that would provide about the same amount of heat, would it be likely to have a similar amount of atmospheric nitrogen as Earth, and, again hypothetically, if life got started there, would it arrive at a similar proportion of atmospheric oxygen in the atmosphere? If so, why, and if not, why not?