generate questions: Free oxygen gas was almost nonexistent in Earth's atmosphere before photosynthetic archaea and bacteria evolved, probably about 3.5 billion years ago. Free oxygen first appeared in significant quantities during the Paleoproterozoic eon (between 3.0 and 2.3 billion years ago). For the first billion years, any free oxygen produced by these organisms combined with dissolved iron in the oceans to form banded iron formations. When such oxygen sinks became saturated, free oxygen began to outgas from the oceans 3–2.7 billion years ago, reaching 10% of its present level around 1.7 billion years ago.
When did photosynthetic organisms evolve on Earth? {sep_token} During which eon did free oxygen begin appearing in quantity? {sep_token} At first, what did oxygen and iron combine to form? {sep_token} How long ago did oxygen reach 10% of its present level? {sep_token} When did oxygen begin to move from the oceans to the atmosphere? {sep_token}