Or nitrogen fixing, the oceans aren't great for nitrogen, being made of water rather than air. Oxygen isn't that scarce at the surface levels, normal waves help

Or nitrogen fixing, the oceans aren't great for nitrogen, being made of water rather than air. Oxygen isn't that scarce at the surface levels, normal waves help with that. Around 40 million years ago when grasses started to take over there was a big resultant change of what went on in the ocean, more nutrients, i.e. fixed nitrogen. This was former 'soil' blown off continents that had gone through a bit of climate change. River sediment too. What happened at the big bang for life 500 million years ago to resolve the nitrogen problem is beyond my wikipedia skills. Also what was the original atmosphere made of, before the oxygen takeover? What I think is amazing is that every rusted or oxidised thing had to get its oxygen from somewhere, i.e. the waste product from micro-organisms getting their carbon. The atmosphere only came about when everything that could be oxidised was oxidised.