The issue of earth's primordial atmosphere features in debates over the origins of life. 3.5 billion years ago, the first prokaryotic organism evolved. It was a simple, unicellular entity similar to the cyanobacteria. Skeptics of evolution think the evolution of life from non-life is impossible because the atmosphere prohibits amino acids and basic organic building blocks from forming a living cell. The presence of free oxygen, specifically, prohibits the production of prebiotic molecules (of the sort Carl Sagan postulated in his theories about a vast primordial ). Of course, the original atmosphere would have been anoxic because the elements of which it was made would consume all available oxygen until much later plants, together with an equilibriated mantle, could provide an oxidizing atmosphere. Thus, the development of prebiotic molecules necessary for abiogenesis (spontaneous generation of life) would not have been governed by this constraint.
Without oxygen, however, ultraviolet light cannot be absorbed, thus reaching the surface and likewise assisting in the destruction of all prebiotics. This oxygen-ultraviolet paradox has been used as an argument for why the earth's early atmosphere could not have been conducive to the evolution of life, and instead would have harshly prohibited it.
Why people would seek grounds on which to challenge a theory all scientists accept as generally close to what actually happened is not immediately evident (especially since most of the arguments are published in misplaced sources, i.e. pamphlets and booklets that are not peer-reviewed or submitted to the scrutiny of a scientific journal); but the answer becomes more obvious when we understand creationism. This is a particular concomitant of religious fundamentalism which takes the Bible as literal truth on all matters, including origins. It's also fueled by an unduly heavy reliance on intuition--even a study which sampled secular-educated high-school students showed they had to hesitate before affirming kinship with ocean-bottom dwelling creatures--which leads them to entertain the absurdity of distrusting scientists and relying on their own logic to tell them evolution defies possibility. Of course, evolution is "staggeringly counter-intuitive" so this isn't surprising . But can't creationists urge their ideas, if they want them to be heard, in scientific forums of debate, so they don't keep claiming evolutionists are "intellectually bankrupt"; (rather, most evolutionists don't even know a significant segment of the lay populace is receptive to anti-evolution propoganda, and most of them don't have time to refute creationist arguments). There is an enormous need for biologically and geologically educated scientists to get together and produce a comprehensive refutation of the arguments advanced against their unifying theory of evolution by the current crop of fundamentalists.
When this happens, perplexed lay persons with the sense to trust the experts can have some proper counter-arguments to anti-evolution objections like the oxygen-ultraviolet paradox.
The answer, so far as I have determined, is that if life really could not have in any conceivable fashion evolved on earth 3.5 billion years ago, then it must have evolved elsewhere. The discipline of astrobiochemistry is devoted to this search for extra-terrestrial prebiotics--so far, the results are encouraging because they demonstrate that basic organic building blocks exist in interstellar gas clouds.