We are protected against lethal ultraviolet rays by a narrow rim of ozone, thirty miles out. We are safe, well ventilated, and incubated, provided we can avoid technologies that might fiddle with that ozone, or shift the levels of carbon dioxide.  Oxygen is not a major worry for us, unless we let fly with enough nuclear explosives to kill off the green cells in the sea; if we do that, of course, we are in for strangling.
It is hard to feel affection for something as totally impersonal as the atmosphere, and yet there it is, as much a part and product of life as wine or bread. Taken all in all, the sky is a miraculous achievement. It works and for what it is designed to accomplish it is as infallible as anything in nature. I doubt whether any of us could think of a way to improve on it, beyond maybe shifting a local cloud from here to there on occasion. The word "chance" does not serve to account well for structures of such magnificence. There may have been elements of luck in the emergence of chloroplasts, but once these things were on the scene, the evolution of the sky became absolutely ordained. Chance suggests alternatives, other possibilities, different solutions. This may be true for gills and swim bladders and forebrains, matters of detail, but not for the sky. There was simply no other way to go.
We should credit it for what it is for sheer size and perfection of function, it is far and away the grandest product of collaboration in all of nature.
It breathes for us, and it does another thing for our pleasure. Each day, millions of meteorites fall against the outer limits of the membrane and are burned to nothing by the friction. Without this shelter, our surface would long since have become the pounded powder of the moon. Even though our receptors are not sensitive enough to hear it, there is comfort in knowing that the sound is there overhead, like the random noise of rain on the roof at night.
      - Lewis Thomas, from “The World's Biggest Membrane”, an essay in The Lives Of A Cell