Our current atmosphere consists primarily of oxygen (21%) and nitrogen (78%) and is called oxidizing because of chemical reactions produced by oxygen. For example, iron is oxidized to form iron oxide or rust.

The presence of oxygen in a hypothetical primordial atmosphere poses a difficult problem for notions of self-assembling molecules. If oxygen is present, there would be no amino acids, sugars, purines, etc. Amino acids and sugars react with oxygen to form carbon dioxide (CO2) and water.

Because it is impossible for life to evolve with oxygen, evolutionists theorize an early atmosphere without oxygen. This departs from the usual evolutionary theorizing where a uniformistic view is held (i.e. where processes remain constant over vast stretches of time). In this case the present is NOT the key to the past.

Instead, they propose a "reducing" (called thus because of the chemical reactions) atmosphere which contains free hydrogen. Originally, they postulated an atmosphere consisting of carbon dioxide (CO2), methane (CH4), carbon monoxide (CO), ammonia (NH3), free hydrogen and water vapor. Newer schemes exclude ammonia and methane.

There is a problem if you consider the ozone (O3) layer which protects the earth from ultraviolet rays. Without this layer, organic molecules would be broken down and life would soon be eliminated. But if you have oxygen, it prevents life from starting. A "catch-22" situation (Denton 1985, 261-262):

In must be noted at this point that the existence of a reducing atmosphere is theoretical and does not rely on physical evidence. To the contrary, there are geological evidences for the existence of an oxidizing atmosphere as far back as can be determined. Among these are: the precipitation of limestone (calcium carbonate) in great quantities, the oxidation of ferrous iron in early rocks (Gish 1972, 8) and the distribution of minerals in early sedimentary rocks (Gish 1984T).

References:

Denton 1985

Gish 1972

Gish 1984T