What would Earth's atmosphere be like if it had remained lifeless?

Imagine life never took hold on Earth? What would its atmosphere be like now? Gas composition, temperature, etc.?

One bump.

Come on, science-oriented Dopers! Surely there’s an answer to this? What would the climate and atmosphere of Earth be like today if life had never existed?

I have no cites, but I’ve heard speculation about a ‘lifeless earth’ atmosphere that was something like 25% carbon dioxide, 74% nitrogen, and other traces including water vapor. Seems slightly feasible as the proportion of biosphere matter that would be gaseous if allowed to return to thermodynamic equilibrium without the effects of life on it.

I’ve read that the Earth’s atmosphere underwent a major change several billion years ago, after the evolution of oxygen-producing organisms. This triggered a shift from a reducing atmosphere to an oxidizing atmosphere. See Paleoclimatology.