Did all the oxygen in the early atmosphere come from simple organisms?

Geologists know from banded iron formations in 2.2 billion-year-old rocks that significant quantities of oxygen were present at the time -- enough, at least, to oxidize the iron in the rocks in a process akin to rusting. Some of that oxygen was presumably generated by photosynthetic cyanobacteria, which were known to exist 3.5 billion years ago, and some came from the chemical separation of water molecules into oxygen and hydrogen.
Source:http://www.scienceagogo.com/news/20000706041735data_trunc_sys.shtml

Thank you, Frankvan, that is a really interesting site. It raises the question of how oxygen could accumulate in the atmosphere to any great extent, because it is so reactive. Once photosynthetic living organisms were on the scene, we can imagine that oxygen was produced at a faster rate than it could combine, so it remained free in the atmosphere. But it is puzzling to think how that could be brought about before life appeared.