*By complex life, I mean something more complex than microorganisms. Something closer to what we have on Earth.
Similar to this question: Is it possible for life to evolve on planets without water?
Is it possible for an atmosphere to sustain life without oxygen? Could life evolve with other gases?
Keeping things to the kind of metabolisms we see on Earth, sure, this is absolutely possible. Even on our oxygen-rich planet we still find obligate anaerobes, to which oxygen is a poison. In fact, your own cells will undergo anaerobic respiration when oxygen-deprived. (This tends to happen after strenuous exercise).
However, respiration with oxygen is much more efficient, approximately 15 times more efficient than anaerobic respiration in your cells. There are some anaerobic organisms which use different electron acceptors than oxygen to narrow this gap a bit.
Wikipedia is a good start for the basics.
There is also the electron transport chain, the summary of which tells more about the role of oxygen.
Yes.
One option would be $\rm F$ (the challenge to overcome is it's relative rarity compared to $\rm O$); or $\rm NF_3$.
A proposed biochemical mechanism I once saw for photosynthesis/respiration cycle could be between $\rm CF_4+N_2+H_2O$ and $\rm NF_3 +$ carbohydrates
There's an extensive discussion of various non-oxygen options (Sulphur, fluorine) in this article on "Exotic Biochemistries" from Xenology Research Institute quotes Carl Sagan, Isaac Asimov and Dr. Peter M. Molton at the University of Maryland, with extensive discussion on alternate biochemistries, including non-oxygen based. The article is very heavy on detail, including analysis of reactivity, energy required to fracture various types of bonds, rarity of elements, possible reactions, etc...
Oxygen breathing is not required to support the life; most of organisms can also use other metabolic pathways to get the energy. Removing all oxygen from the Earth atmosphere would be apocalyptic of course, with most of animals dying immediately, however some plant species may survive on anaerobic breathing during the night (plants produce plenty of their own oxygen as long as photosynthesis is running). Anaerobic bacteria already living in oxygen-less places of Earth would not notice anything wrong happening and will take over leading the evolution.
Disappearance of the ozone layer will increase the UV radiation. However this work shows that sea water (with usual particles in it) also provides efficient protection, so water organisms should probably survive, even if affected. UV-proof shells and scales may evolve.
However oxygen is also an important part of protein, DNA and cell membranes that are all very essential components of every living cell, as well as water that is also essential. Nitrogen and fluorine are not replacements, these elements have very different properties, same about hydrogen. Sulfur is somewhat similar and may substitute oxygen in some degree, but all molecular composition of life must be redesigned.
Certainly. We already know of different ways to produce energy without oxygen and organisms that don't require oxygen or are even harmed by it. So you definitely can have life without oxygen.
However, how complex can that life be? Oxygen powers the citric acid cycle, which produces an average of 30 units of energy, (ATP), compared to anaerobic which can produce about 2. This is a massive energy jump. Without oxygen, I don't see any large multicellur organism developing that can move quickly under its own power.
Additionally, without oxygen and the ozone it forms, your world would be at the mercy of ultraviolet light. This light can break chemical bonds necessary for life and damage DNA. Without oxygen to form a cover, life has a much harder, maybe even impossible time to survive. Life would have to remain under rocks or far from the light of the sun to survive.
Sure! We already got some examples, just do a little research for Anaerobic Organism. Good point to start: http://en.wikipedia.org/wiki/Anaerobic_organism
No Oxygen is really not a big issue I think, but no carbon would be one. Even, if there are theorys of silicon based life, instead of carbon based.
This would not be possible on the external circumstances as we have them here on the earth (temperature, pressure,...) due silicon is solid matter here, but why couldn't be life on a 500°C survace-planet on silicon base?
If you think about that, you can not say that higher life without oxygen is inpossible.
