Could a terrestrial planet have a stable set of layers of atmospheres, and could different lifeforms evolve within those different layers?
This question is inspired by the diversity found on Earth. For example, deep sea organisms evolved without light (and in some cases, without oxygen) and are quite different from other organisms at the surface, which in turns are quite different from flying or walking species.
To clarify, I was thinking about a kind of swamp, where land-like creatures and plants living below a certain altitude would evolve in a different atmosphere. I though that maybe the gas could be "trapped" in a valley or isolated from the upper layer by the surrounding vegetation.
You may have guessed that I am far from being a biologist.
The question popped in my head while thinking about a possible science-fiction story. Tim has good arguments about why this is not likely to happen and I appreciate the reality-check. I also doubt that the kind kind of gas that could support life (See "Is it possible for complex life to evolve on planets without oxygen?") would also be heavier than, say, oxygen, and form a distinct layer.
But nonetheless, if you can find creative yet plausible ways about how it could happen, please share your thoughts. For example, I like the idea that a more massive planet might be suitable for the existence of multiple layers, as well as the possibility that weird forms of life can evolve in that kind of environment (see HDE 226868's answer).